clutter-actor: Add detail to captured-event signal [performance]

The detail allows handlers to distinguish between high volume cursor motion events and everything else. This allows (most) event handlers who don't care about cursor motion to not have their power/CPU usage impacted by it.

Using minimal classification is important because it allows those handlers who do want to use this optimization via captured-event::nonmotion to keep supporting multiple event types without requiring any logic changes.

Any handler that does want cursor motion events and others should simply remain unmodified and keep using captured-event.

When combined with gnome-shell!276 (closed) this helps to solve #283 (closed). Although see also !189 (merged) which provides a much bigger improvement.