Add better deprecation macros

Unlike G_GNUC_... macros, the new G_DEPRECATED[_FOR] are
meant as abstractions that work with different compilers.
Using a new name also lets us restrict it to 'must be placed
before the declaration', which works with more compilers.

https://bugzilla.gnome.org/show_bug.cgi?id=661438

docs/reference/glib/glib-sections.txt

@@ -362,6 +362,10 @@ G_HAVE_GNUC_VISIBILITY
 G_GNUC_INTERNAL
 G_GNUC_MAY_ALIAS
 
+<SUBSECTION>
+G_DEPRECATED
+G_DEPRECATED_FOR
+
 <SUBSECTION>
 G_LIKELY
 G_UNLIKELY