[GTK] Move drag-and-drop code which can be shared with WebKit2 to WebCore

[GTK] Move drag-and-drop code which can be shared with WebKit2 to WebCore
https://bugs.webkit.org/show_bug.cgi?id=66890

Source/WebCore:

Added a GtkDragAndDropHelper class to hold all the logic that is common
between WebKit1 and WebKit2. This will allow greater code sharing between
the two ports.

Reviewed by Philippe Normand.

No new tests. This patch should not change behavior.

* GNUmakefile.list.am: Added the GtkDragAndDropHelper to the sources list.
* platform/gtk/GtkDragAndDropHelper.cpp: Added.
* platform/gtk/GtkDragAndDropHelper.h: Added.

Source/WebKit/gtk:

Use the new GtkWidgetDragAndDropGlue class to hold the logic that will be
common between WebKit1 and WebKit2. Modify WebKitWebView to use this class.

Reviewed by Philippe Normand.

* WebCoreSupport/DragClientGtk.cpp:
(WebKit::DragClient::startDrag): Inform the drag-and-drop glue about the drag.
* webkit/webkitwebview.cpp: Use GtkWidgetDragAndDropGlue wherever this class was
maintaining drag-and-drop state itself.
* webkit/webkitwebviewprivate.h: Swap out the drag-and-drop state for an instance
of the glue class.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96299 268f45cc-cd09-0410-ab3c-d52691b4dbfc