Give each image window a dedicated status bar

(Relates to #106)

While I generally do not like persistent chrome - particularly chrome
that eats up vertical space - but there's no denying that this is going
to make my life much easier, and hopefully improve program usability as
well.

I plan on moving a number of on-screen elements to this new status bar -
mouse coords, zoom, and messages, to start.  I think this will be a
better solution than having one copy of each, as persistence is
important - particularly in "floating window" mode.  It will also
simplify my code to simply associate each image with a single copy of
things like a zoom box and mouse coordinate display.

This will also allow for potential horizontal controls for these
actions, like a slider control for zoom rather than the current
drop-down.