Fullscreen windows should be above _everything_, so keepBelow should be false.

Prevents .xsession-errors from getting big.

This is much cleaner than the hodgepodge we had before
and also means e.g. desktopswitching can be handled correctly.

It is now effectively a layout.

This requires a semantics change: addClient can now also deal with already managed clients.

This also fixes a bug with tiling because a method was missing.