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/gtklayout.c

@@ -32,7 +32,7 @@
 
 #include "gtklayout.h"
 
-#include "gdkconfig.h"
+#include "gdk/gdk.h"
 
 #include "gtkprivate.h"
 #include "gtkintl.h"