Add MetaGravity and replace X11 equivalent with it

MetaGravity is an enum, where the values match the X11 macros used for
gravity, with the exception that `ForgetGravity` was renamed
`META_GRAVITY_NONE` to have less of a obscure name.

The motivation for this is to rely less on libX11 data types and macros
in generic code.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/705

src/core/constraints.h

@@ -29,7 +29,7 @@
 
 void meta_window_constrain (MetaWindow          *window,
                             MetaMoveResizeFlags  flags,
-                            int                  resize_gravity,
+                            MetaGravity          resize_gravity,
                             const MetaRectangle *orig,
                             MetaRectangle       *new,
                             int                 *rel_x,