Remove stumpwmContrib from top-level and fix runtime dependency on it

This patch removes the stumpwmContrib package from the top-level since
it can't sensibly be used on its own. Also, it wraps the stumpwm
executable with a dummy reference to the contrib dir to work around the
issue that the stumpwm executable doesn't reference the contrib dir
that's passed in the configure phase for some reason.
Better work-around for stumpwmContrib dependency