Move decoration renderer to decoration item

Currently, the implementation of the DecoratedClient and the decoration renderer are strongly coupled. This poses a problem with the item based design as the ultimate goal is to have scene items construct paint nodes which are then fed to the renderer. The DecorationItem has to have control over the decoration texture. Another issue is that the scene cannot smoothly cross-fade between two window states if the decoration is removed, e.g. from fullscreen mode to normal and vice versa.

This change moves the decoration renderer to the decoration item. With the introduction of a generic scene texture atlas, we hope to get rid of the decoration renderer altogether.