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update monitor bounds on each layout()
Hey look, it's multiple monitor support (fixes #21)

Tested on gs-3.10, where window.get_monitor() seems to work correctly
now for newly-created windows. There are occasional glitches when
moving a window onto the secondary monitor, but most of the time it
works fine.

When testing, I crashed mutter a few times. It's mutter's fault, but I
can't reproduce again to figure out if there's some workaround I could
do. Fingers crossed?