Merge libgdk and libgtk

This commit does a number of things:
- remove some dead wchar configury from configure.ac and gdkconfig.h
- repurpose gdkconfig.h as header that contains GDK_WINDOWING_foo
  macros for each included backend, include it in gdk.h and install
  it in $includedir instead of below $libdir
- drop the backend from the library names
- build libgdk-3.0.la as a convenience lib and include it in libgtk-3.0.la
It does not yet enable building multiple backends at the same time.

gtk/gtkliststore.h

@@ -24,7 +24,6 @@
 #ifndef __GTK_LIST_STORE_H__
 #define __GTK_LIST_STORE_H__
 
-#include <gdkconfig.h>
 #include <gtk/gtktreemodel.h>
 #include <gtk/gtktreesortable.h>