Fix ContextMenu allocation in the Qt port.

Fix ContextMenu allocation in the Qt port.

Store all items and submenus value based in ContextMenu and ContextMenuItem.
That fixes the crashes when the context menu was populated with sub-menus because
of the use of temporary ContextMenu objects like this:

ContextMenu subMenu(...);
subMenu.appendItem(...);
subMenu.appendItem(...);

subMenuItem.setSubMenu(&subMenu); // temporary pointer, need to _copy_ contents

Signed-off-by: Holger
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@27644 268f45cc-cd09-0410-ab3c-d52691b4dbfc