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.

modules/other/gail/Makefile.am

@@ -141,8 +141,7 @@ libgail_la_CFLAGS = \
 	$(AM_CFLAGS)
 
 libgail_la_LIBADD =  \
-	$(top_builddir)/gdk/$(gdktargetlib) 				\
-	$(top_builddir)/gtk/$(gtktargetlib)				\
+	$(top_builddir)/gtk/libgtk-3.0.la				\
 	$(top_builddir)/modules/other/gail/libgail-util/libgailutil-3.0.la	\
 	$(GTK_DEP_LIBS)							\
 	$(INTLLIBS)