WaylandExecutor should implement time::AlarmFactory (and probably the rest of MainLoop?)

[RAOF]

The Wayland event loop pretty much implements our MainLoop interfaces - timers, signal watches, fd watches. When interacting with Wayland it's almost always necessary to do so from the Wayland event loop, so currently when we want to watch a file descriptor and then do something on the Wayland loop, or set a timer to do something on the Wayland loop we need to first set the timer, and then from the callback push another callback into the WaylandExecutor that can then be run on the Wayland loop.

Instead we could just have a Wayland-main-loop-backed implementation of MainLoop that does everything on the event loop, making everything cleaner.