core: Untangle input focus management

In all places (including src/wayland) we tap into meta_x11_display* focus
API, which then calls meta_display* API. This relation is backwards, so
rework input focus management so it's the other way around.

We now have high-level meta_display_(un)set_input_focus functions, which
perform the backend-independent maintenance, and calls into the X11
functions where relevant. These functions are what callers should use.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/420

src/core/window.c

@@ -8088,8 +8088,7 @@ mouse_mode_focus (MetaWindow  *window,
                       "Unsetting focus from %s due to mouse entering "
                       "the DESKTOP window\n",
                       display->focus_window->desc);
-          meta_x11_display_focus_the_no_focus_window (display->x11_display,
-                                                      timestamp);
+          meta_display_unset_input_focus (display, timestamp);
         }
     }
 }