Refactor autohide

[wash2]

It doesn't seem to be getting notified of the overlaps even when I add the global to cosmic-comp. I'll have to look more into the cosmic-comp implementation.

[Drakulix]

Looks good, though we probably would want to merge this after the cosmic-comp patches to not temporarily regress the autohide behaviour.

[git-f0x]

Not sure if it's intended, but the panels can no longer draw outside their revealed-state container, so they just get cropped during the autohide animation (most visible with floating and rounded panels).
Also, after opening many apps and then closing them, the center segment applet positions seem to remain at the offset from the center caused by the many app icons (the segment position isn't reset).

Edit: The latter doesn't seem to be a regression, since I can also reproduce it on the current build (not sure if it's related to overflow).