Basic wayland backend input

This implements pointer/keyboard input support for the wayland backend. Touch is not supported yet in any way (cannot test it, probably not as important). Axis support for pointers could also be improved and it does currently not handle the received keymap (there is no way to do it, xkbcommon would have to be handled by the library). Due to the current pointer event api, pointer movement events are only handled relatively (not sure about this, should it use wlr_event_pointer_motion_absolute and just imagine some fake mm values?).