[GTK] Allow pinch zoom on touchpad

[GTK] Allow pinch zoom on touchpad
https://bugs.webkit.org/show_bug.cgi?id=194201

Patch by Alexander Mikhaylenko <exalm7659@gmail.com> on 2019-02-04
Reviewed by Michael Catanzaro.

Enable touchpad events for WebkitWebViewBase, then feed touchpad
pinch events to GestureController.

* UIProcess/API/gtk/WebKitWebViewBase.cpp:
(webkitWebViewBaseRealize): Added GDK_TOUCHPAD_GESTURE_MASK to event mask.
(webkitWebViewBaseEvent): Added.
(webkit_web_view_base_class_init): Override event vfunc.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@240922 268f45cc-cd09-0410-ab3c-d52691b4dbfc