st-clipboard: Use GDK for the event filters instead of Clutter

st-clipboard: Use GDK for the event filters instead of Clutter

Instead of using Clutter to add an event filter for X events it now
uses the GDK API. The Clutter API won't work if Clutter is not using
an X11-based backend such as if Mutter is directly running with the
KMS backend. This is a step towards making Mutter be its own display
server and a step towards being a Wayland compositor. In this case GDK
will still be using the X backend because it will connect to the
headless X server.

https://bugzilla.gnome.org/show_bug.cgi?id=693438