x11: Simplify handling of focus-follows-mouse

Focus follows mouse is meant to avoid focusing windows that happened to pop up under the pointer, e.g. due to mapping, workspace changes, etc... On X11, this has been done since ancient times through a moderately complex synchronization mechanism, so mutter would know to ignore crossing events caused on those situations.

Drop the 2 serial checking mechanisms in place, and resort to a check for non-motion-triggered crossing events through XInput2 (introduced after these mechanisms) based on the source device not being a hardware one. This is also closer to the existing checks on Wayland.