Clean up

Rename `bar` to `barwin`, to make clear we speak of an X Window object.
Also fix a few unnecessary newlines, comments, and missing function
prototypes, along with a more friendly handling of interrupts by
select().
Add master module

The master module will have additional capabilities that make it able to
also modify the behaviour of the overall karuibar (and not only its own
data/pixmap area).
Bar visibility can be toggled now

This is also available to the master module instroduced in the last
commit.

The signal SIGUSR2 will toggle the visibility. Also, instead of SIGHUP,
SIGUSR1 is now used to restart the bar (so if at the next invocation of
`pkill -HUP karuibar` the bar disappears: that's intended).