window: Make meta_window_can_tile_side_by_side() public

In order to support keybindings for window tiling, we need to
determine whether a window is tilable or not, so make this public.

https://bugzilla.gnome.org/show_bug.cgi?id=648700

src/core/window-private.h

@@ -653,5 +653,6 @@ void meta_window_propagate_focus_appearance (MetaWindow *window,
                                              gboolean    focused);
 
 gboolean meta_window_should_attach_to_parent (MetaWindow *window);
+gboolean meta_window_can_tile_side_by_side   (MetaWindow *window);
 
 #endif