Add wlr_input_device_is_foo and wlr_output_is_bar functions
This adds all the variants of wlr_input_device and wlr_output that I could find that have and impl.
This last point is important, because X11 has wlr_input_devices with a NULL implem, so we can't use that to check as we have done with backends. I've considered creating a dummy impl initialized to 0 to use that just for check, should I?
Closes #482 (closed). Reopens #497 (closed). (I'm not sure that's legit, but these really are two separate issues. I doubt I'll forget but feels cleaner with the issue open)