Remember window size

This commit has gone through a few iterations and I trimmed it down to
what I consider the "conservative minimum" - so this only stores window
size, not window position. And in my mind that's the more relevant part,
anyway. Have your window manager position the window at a "smart" spot on
your screen...

Signed-off-by: Amit Chaudhuri <amit.k.chaudhuri@gmail.com>
Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add MacOS support to window geometry save/restore

Also make sure the config is flushed to disk.

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>