Fixes#2462
This sets up a listener for screen parameter changes. This only triggers
when a screen is added, removed, or a parameter such as its resolution
changes. This doesn't trigger when a window is simply moved from one
screen to another.
On parameter change, we ensure that the window is within the bounds of
the screen. As an exception, if the window was previously already
outside the bounds of the screen, we don't move it back in.
