Fix #29 by simplifying mouse button handling

Pressing two mouse buttons at the same time didn't work properly, only
one had effect, e.g. when pressing LMB to walk forward and RMB to shoot
(when bound like that...)

Not sure why that was, but the old mouse button handling (inherited from
the super-old win32 backend where it was supposed to work around bugs in
WinAPI or whatever, where sometimes there were two mouse button presses
in one event or something) was unnecessarily complicated anyway so I
replaced it with something simpler.