Focus nearest window when clicking on background

[neunato]

With gaps enabled, this would make it possible to navigate left/right in the workspace by clicking on the very edge of the screen (when windows don't overflow on edges).

Also works around the problem discussed here #984 when clicking on top/bottom gaps.

[YaLTeR]

Hmm, interesting idea. Need to try to see if it actually works well in practice.

Also it might not work if you're running some desktop icons thing since it'll be intercepting the clicks.