Screen Edges may belong to fullscreen windows

Corners are still ours (it's a valid use case to still be able to switch
window through e.g. Present Windows even when running a fullscreen app).

How is it done? An Edge can be blocked and does no longer trigger if it
is blocked. For WindowBasedEdges the edge windows get unmapped in the
blocking case and mapped again when the blocking condition is no longer
valid.

The blocking is so far connected to:
* changes of active window
* changes of fullscreen windows

Whenever one of the events occurs it is checked whether there is:
1. an active client
2. it is fullscreen
3. on the same screen as the edge

If this is the case the edge will be blocked, otherwise unblocked.


FIXED-IN: 4.11