wayland: Add MetaWaylandKeyboardGrab and keyboard grab API

This will be useful during DnD, where mutter is expected to consume
keyboard events for either allowing changes in the selected DnD action,
or misc a11y features like keyboard-driven DnD.

Currently, the vtable contains 2 functions, key() will be used on every
key event we get from Clutter, modifiers() will notify of changes in the
keyboard modifiers (mouse buttons will never be set in the modifier mask)

https://bugzilla.gnome.org/show_bug.cgi?id=760805