clutter: add keyboard accessibility signals

Implementing keyboard accessibility in clutter means we need to be able
to notify higher layers of possible modifier masks or setting changes
(as some keyboard accessibility can be disabled or enabled by keyboard).

For this purpose, add two new signals:

   ClutterDeviceManager::kbd-a11y-mods-state-changed
   ClutterDeviceManager::kbd-a11y-flags-changed

https://bugzilla.gnome.org/show_bug.cgi?id=788564