gtk3: gtk.h includes gtk-2.0 gdk.h

Proper operation of the gtk-3.0 headers depends on not bringing in the
gtk-2.0 headers, because somewhere we'll get some kind of conflicts.
However, the 2.0 gdk.h header is included due to several types. Here's
the debug output from building gtk-3.0 gtk.h, which is the only header
which includes a gtk-2.0 header:

/* DEBUG: type=12741 (GdkAtom) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12912 (GdkGravity) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12856 (GdkWindowTypeHint) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12730 (GdkEventMask) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12709 (GdkModifierType) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12916 (GdkWindowHints) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12836 (GdkWindowEdge) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12912 (GdkGravity) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12806 (GdkDragAction) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12804 (GdkDragProtocol) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12730 (GdkEventMask) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12709 (GdkModifierType) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12741 (GdkAtom) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12856 (GdkWindowTypeHint) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12708 (anon-gtk-2.0/gdk/gdk.h-14) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12729 (anon-gtk-2.0/gdk/gdk.h-934) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12740 (_GdkAtom *) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12803 (anon-gtk-2.0/gdk/gdk.h-496) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12805 (anon-gtk-2.0/gdk/gdk.h-495) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12835 (anon-gtk-2.0/gdk/gdk.h-954) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12855 (anon-gtk-2.0/gdk/gdk.h-950) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12911 (anon-gtk-2.0/gdk/gdk.h-953) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12915 (anon-gtk-2.0/gdk/gdk.h-949) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12774 (GdkEventKey) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12772 (GdkEventScroll) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12793 (GdkEventOwnerChange) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12865 (GdkPoint *) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12864 (GdkPoint) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12769 (_GdkEventScroll) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12773 (_GdkEventKey) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12790 (_GdkEventOwnerChange) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12863 (_GdkPoint) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12748 (GdkEventType) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12674 (GdkWindow *) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12771 (GdkScrollDirection) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12713 (GdkDevice *) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12788 (GdkNativeWindow) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12792 (GdkOwnerChange) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12673 (GdkWindow) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12712 (GdkDevice) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12747 (anon-gtk-2.0/gdk/gdk.h-501) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12770 (anon-gtk-2.0/gdk/gdk.h-504) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12791 (anon-gtk-2.0/gdk/gdk.h-509) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12672 (_GdkDrawable) found in gtk-2.0/gdk/gdk.h */
/* DEBUG: type=12697 (_GdkDevice) found in gtk-2.0/gdk/gdk.h */

At the time of filing, the full list of these types have not been
investigated to see if they are indeed defined both places. However,
here's a spot check:

INSERT INTO `Type` VALUES
(12673,'GdkWindow','Typedef',626,'','No','No','No',NULL,'libgdk-x11-2.0',0);
INSERT INTO `Type` VALUES
(12712,'GdkDevice','Typedef',626,'','No','No','No',NULL,'libgdk-x11-2.0',0);
INSERT INTO `Type` VALUES
(12864,'GdkPoint','Typedef',626,'','No','No','No',NULL,'libgdk-x11-2.0',0);
INSERT INTO `Type` VALUES
(10004007,'GdkPoint','Typedef',1795,'','No','No','No',NULL,'libgdk-3',0);
INSERT INTO `Type` VALUES
(10004018,'GdkDevice','Typedef',1795,'','No','No','No',NULL,'libgdk-3',0);
INSERT INTO `Type` VALUES
(10004030,'GdkWindow','Typedef',1795,'','No','No','No',NULL,'libgdk-3',0);

which suggests that while new types have been defined, some definitions
within the library relating to this header have picked up the original
type definition.