Draft: Implement 2D gestures

This allows:

• Switching diagonally between virtual desktops
• Moving in the overview up/down and switching between desktops left/right at the same time

This almost works. Issue is that this makes it almost impossible to open the Overview, since in most cases it will trigger a left/right virtual desktop changing animation first, and the overview refuses to open if there's another effect open already.

output

This MR's parent is the Overview MR, as that's required for this work. Thus the mess in the diff. As soon as the overview is merged it's going to be much smaller, see commit for reference.