wayland: Set/unset wayland focus on mutter grab/ungrab operations

This ensures that we send the proper leave and enter events to wayland
clients.

Particularly, this solves a bug in SSD xwayland windows where clicking
and dragging on the title bar to move the window only works on the odd
turn (unless the pointer moves away from the title bar between
tries). This happens because xwayland gets a button press but doesn't
see the release so when it gets the next button press it discards it
because its pointer button tracking logic says that the button is
already pressed. Sending the proper wayland pointer leave event fixes
it since wayland clients must forget about button state at that point.

https://bugzilla.gnome.org/show_bug.cgi?id=726123

src/core/display-private.h

@@ -486,4 +486,6 @@ void meta_display_set_input_focus_xwindow (MetaDisplay *display,
                                            Window       window,
                                            guint32      timestamp);
 
+void meta_display_sync_wayland_input_focus (MetaDisplay *display);
+
 #endif