[GTK] Rework the theming code for GTK+ 3.20

[GTK] Rework the theming code for GTK+ 3.20
https://bugs.webkit.org/show_bug.cgi?id=156333

Reviewed by Michael Catanzaro.

.:

Add a manual test to check how themed elements are rendered.

* ManualTests/gtk/theme.html: Added.

Source/WebCore:

During the 3.19 GTK+ release cycle, the GTK+ css system was reworked, making themes and programs rendering
themed widgets, incompatible with the new system. We were trying to fix our rendering every time GTK+ broke
something, but we were just changing whatever it was needed to make our rendering look like current GTK+ with
the default theme Adwaita. This means that our rendering will be broken for other themes or that changes in
Adwaita can break our rendering. This solution was good enough to ensure WebKitGTK+ 2.12 looked good with GTK+
3.20, but it doesn't work in the long term. We need to ensure that our theming code honors the new GTK+ CSS
properties (max-width, min-width, margin, padding, border, ...) in all the cases, not only the cases where
Adwaita uses them like we currently do.
This patch splits all rendering methods to keep the current code for previous GTK+ versions and adds new code
for GTK+ >= 3.20 using the new RenderThemeGadget classes. This makes the code easier to read, since there aren't
ifdef blocks in the functions, and we ensure we don't break previous rendering.

* PlatformGTK.cmake: Add new files to compilation.
* html/shadow/SpinButtonElement.cpp:
(WebCore::SpinButtonElement::defaultEventHandler): Check the button layout used by the theme to decide the
current buttons state.
* platform/gtk/RenderThemeGadget.cpp: Added.
(WebCore::RenderThemeGadget::create):
(WebCore::createStyleContext):
(WebCore::appendElementToPath):
(WebCore::RenderThemeGadget::RenderThemeGadget):
(WebCore::RenderThemeGadget::~RenderThemeGadget):
(WebCore::RenderThemeGadget::marginBox):
(WebCore::RenderThemeGadget::borderBox):
(WebCore::RenderThemeGadget::paddingBox):
(WebCore::RenderThemeGadget::contentsBox):
(WebCore::RenderThemeGadget::color):
(WebCore::RenderThemeGadget::backgroundColor):
(WebCore::RenderThemeGadget::minimumSize):
(WebCore::RenderThemeGadget::preferredSize):
(WebCore::RenderThemeGadget::render):
(WebCore::RenderThemeGadget::renderFocus):
(WebCore::RenderThemeBoxGadget::RenderThemeBoxGadget):
(WebCore::RenderThemeTextFieldGadget::RenderThemeTextFieldGadget):
(WebCore::RenderThemeTextFieldGadget::minimumSize):
(WebCore::RenderThemeToggleGadget::RenderThemeToggleGadget):
(WebCore::RenderThemeToggleGadget::render):
(WebCore::RenderThemeArrowGadget::RenderThemeArrowGadget):
(WebCore::RenderThemeArrowGadget::render):
(WebCore::RenderThemeIconGadget::RenderThemeIconGadget):
(WebCore::RenderThemeIconGadget::gtkIconSizeForPixelSize):
(WebCore::RenderThemeIconGadget::render):
(WebCore::RenderThemeIconGadget::minimumSize):
* platform/gtk/RenderThemeGadget.h: Added.
(WebCore::RenderThemeGadget::context):
* rendering/RenderTheme.h:
(WebCore::RenderTheme::innerSpinButtonLayout): Added this method to allow themes use a different layout for the
buttons.
* rendering/RenderThemeGtk.cpp:
(WebCore::themeChangedCallback): Just moved this code to a common place.
(WebCore::RenderThemeGtk::RenderThemeGtk): Initialize the theme monitor in the constructor.
(WebCore::createStyleContext): Remove the render parts that are specific to GTK+ 3.20.
(WebCore::RenderThemeGtk::adjustRepaintRect): Moved inside a GTK+ < 3.20 ifdef block.
(WebCore::themePartStateFlags): Helper function to get the GtkStateFlags of a theme part for a given RenderObject.
(WebCore::shrinkToMinimumSizeAndCenterRectangle): Move this common code to a helper function.
(WebCore::setToggleSize):
(WebCore::paintToggle):
(WebCore::RenderThemeGtk::paintButton):
(WebCore::RenderThemeGtk::popupInternalPaddingBox):
(WebCore::RenderThemeGtk::paintMenuList):
(WebCore::RenderThemeGtk::adjustTextFieldStyle): For GTK+ 3.20 we need to ensure a minimum size for spin buttons,
so if the text field is for a spin button, we adjust the desired size here.
(WebCore::RenderThemeGtk::paintTextField): In GTK+ 3.20 the CSS gadgets used to render spin buttons are
different, so we check here if this is the entry of a spin button to use the right gadgets.
(WebCore::adjustSearchFieldIconStyle):
(WebCore::RenderThemeGtk::paintTextArea):
(WebCore::RenderThemeGtk::adjustSearchFieldResultsButtonStyle):
(WebCore::RenderThemeGtk::paintSearchFieldResultsButton):
(WebCore::RenderThemeGtk::adjustSearchFieldResultsDecorationPartStyle):
(WebCore::RenderThemeGtk::adjustSearchFieldCancelButtonStyle):
(WebCore::paintSearchFieldIcon):
(WebCore::RenderThemeGtk::paintSearchFieldResultsDecorationPart):
(WebCore::RenderThemeGtk::paintSearchFieldCancelButton):
(WebCore::centerRectVerticallyInParentInputElement): Moved inside a GTK+ < 3.20 ifdef block.
(WebCore::RenderThemeGtk::paintSliderTrack):
(WebCore::RenderThemeGtk::adjustSliderThumbSize):
(WebCore::RenderThemeGtk::paintSliderThumb):
(WebCore::RenderThemeGtk::progressBarRectForBounds): Ensure a minimum size of progress bars in GTK+ 3.20.
(WebCore::RenderThemeGtk::paintProgressBar):
(WebCore::RenderThemeGtk::innerSpinButtonLayout): Use an horizontal layout for spin buttons.
(WebCore::RenderThemeGtk::adjustInnerSpinButtonStyle):
(WebCore::RenderThemeGtk::paintInnerSpinButton):
(WebCore::styleColor):
(WebCore::RenderThemeGtk::paintMediaButton):
* rendering/RenderThemeGtk.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@199292 268f45cc-cd09-0410-ab3c-d52691b4dbfc