gtk: Empty gtkimmodule.h

Move the remaining struct definition into gtkimcontextinfo.h and include
that header in gtk.h. gtkimmodule.h is now an empty header. We should
probably deprecate it somehow.

This is also necessary so headers used in gtk .c files don't include
gtk.h which in turn includes all the deprecated headers which we want to
avoid so we can include them with deprecation warnings turned off.

gtk/gtkimmulticontext.c

@@ -23,7 +23,6 @@
 #include <locale.h>
 
 #include "gtkimmulticontext.h"
-#include "gtkimmodule.h"
 #include "gtkimmoduleprivate.h"
 #include "gtklabel.h"
 #include "gtkmain.h"