App/layers: deactivate surfaces on layers above.

App/layers: deactivate surfaces on layers above.

* When activating a surface on layer x, all surfaces on all layers above
  this will be deactivated.
* Remove main_surface special case (we get the same result by putting
  the main_surface on the bottom layer).
* Track LayoutState per layer, make can_split() accept this layer-local
  LayoutState for its decision.
* Sort layers on load according to their ID. That is, do not make use
  Of the layer's z-order attribute, also there is no need to.

Signed-off-by: Marcus Fritzsch <marcus_fritzsch@mentor.com>