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.

gdk/tests/Makefile.am

@@ -12,7 +12,7 @@ AM_CPPFLAGS = \
 
 progs_ldadd = \
 	$(GDK_DEP_LIBS) \
-	$(top_builddir)/gdk/libgdk-$(gdktarget)-$(GTK_API_VERSION).la \
+	$(top_builddir)/gdk/libgdk-$(GTK_API_VERSION).la \
 	$(NULL)
 
 #TEST_PROGS              += check-gdk-cairo