keyboard: Add KeyboardManager to manage the lifetime of the keyboard actor

keyboard: Add KeyboardManager to manage the lifetime of the keyboard actor

The Keyboard class used to be both a view and controller class, however in order
to make the keyboard a native Clutter.Actor, we need to separate the widget from
the controller class, so that we can manage the actor lifetime from the JS side.

Thus, initialize the keyboard actor on the Keyboard constructor and create a
KeyboardManager class to manage its state and lifetime.

Add proxy methods for the public functions that were used by other shell
components

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/559