* Qt and Gtk must know if a ContextMenuItem is checkable. Add a new ContextMenuItemType for checkable
  actions.
* Use this information in the Gtk platform to create a GtkCheckMenuItem when needed.
* Update the ContextMenuController to accept CheckableActionTypes as well.
* Change ContextMenu.cpp to use the CheckableActionType. The information if a item is checkable
  was extracted from ContextMenu::checkOrEnableIfNeeded.
* Update the Qt and Windows port.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@29230 268f45cc-cd09-0410-ab3c-d52691b4dbfc