Implement DisplayRefreshMonitor* classes

Add the initial DisplayRefreshMonitor support. This enables us to
rely on Wayland's frame callbacks (basically vsync) to fire off
requestAnimationFrame callbacks and update running animations.

Previously we relied on dumb timers for this, which was not ideal.
Still, the current implementation is only an initial one, and does
not address all the corner cases (i.e. multiple screens, future
threaded compositing etc.).
Rely on display monitor for rescheduling animation-caused flushes.
Remove an unneeded member variable in LayerTreeHostWPE.