MetaWaylandKeyboard: use the new clutter-evdev support for xkbcommon

We need to track the full xkb_state to have the necessary information
to send to the clients, otherwise they may get confused and lock
or invert the modifiers. In the evdev backend, we just retrieve the
same state object that clutter is using, while in the other backends
we fake the state using what clutter is providing (which is a subset
of what X11 provides, which would be necessary to have full state)

https://bugzilla.gnome.org/show_bug.cgi?id=705862
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
wayland: don't free surfaces that have a window associated

After a MetaWaylandSurface is associated with a MetaWindow, it
should be freed only when the MetaWindow is unmanaged. For wayland
clients, the window is unmanaged when the resource is destroyed,
but for X11 clients we want to wait for the unmap event.

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