display: move more event handling to clutter

Mouse event handling was duplicated, resulting in weird interactions
if clutter was allowed to see certain events (for example under
wayland, where it gets all events). Because now clutter sees all
X events, even when running as an x11 compositor, we can handle
everything using the clutter variants.
At the same time, rewrite a little the passive button grab code,
to make it clear what is being matched on what and why.