Make it possible to dock windows at the edges of the monitor

[hedning]

The idea is to reduce the effective monitor size by the width of the docked window when positioning and resizing the tiled windows.

The docked window should probably be styled like a tiled window, ie. maximized vertically with square corners. It's not obvious if we want a margin between the edge of the docked window and the tiling as might look weird:

But the visual effect might be somewhat subtle without a margin (and means you can't always click on the next window):