wayland: implement support for plugin modality

Calling XIGrabDevice has no effect under wayland, because the
xserver is getting events from us. Instead, we need to use our
own interfaces for grabs.
At the same time, we can simplify the public API, as plugins
should always listen for events using clutter.

https://bugzilla.gnome.org/show_bug.cgi?id=705917

src/wayland/meta-wayland-keyboard.h

@@ -70,6 +70,13 @@ meta_wayland_keyboard_start_grab (MetaWaylandKeyboard *device,
 void
 meta_wayland_keyboard_end_grab (MetaWaylandKeyboard *keyboard);
 
+gboolean
+meta_wayland_keyboard_begin_modal (MetaWaylandKeyboard *keyboard,
+				   guint32              timestamp);
+void
+meta_wayland_keyboard_end_modal   (MetaWaylandKeyboard *keyboard,
+				   guint32              timestamp);
+
 void
 meta_wayland_keyboard_release (MetaWaylandKeyboard *keyboard);