Introduce Decoration::damaged signal

This signal is needed to support various server decoration renderers. In kwin, we'd like to get rid of the DecorationRenderer in the composited code path in favor of a unified texture atlas. However, on X11, we still need to support the code path where compositing is off.

This change introduces the Decoration::damaged() signal to make server side decoration rendering machinery in the compositor more flexible. The new signal is similar to the SurfaceInterface::damaged() signal.