compositor: Layers

Add a bunch of items to serve as layers where we put surfaces.

One of the things I like most of Qt based compositor is that you can
easily implement layers with items with the proper z-index value.

Surface items just need to be created as children of the layer items
to get a propert stacking order.

Issue: #148
compositor: Implement window stacking with QML

Move window stacking code to QML.
Temporarily remove animations until the refactoring is complete.

Issue: #148
compositor: Use Green Island wrappers for windows
compositor: Emit signals when windows are added, removed or destroyed
compositor: Don't send frame callbacks

Green Island already take care of it.