treat startup command as long-running

Not quite a perfect mirror of xinit, where the startup command execs the
window manager, and the termination of that process brings down the
windowing system, but it might be the Wayland analogue.

fix typo in comment

implement Button: movemouse, resizemouse

add CLEANMASK like dwm

make key mappings configurable in config.h

start factoring Key actions into functions

Start with tinywl

Add a config.h into which configurables can be moved.