[kstyle] Implement window shadows on Wayland
Review Request #127390 - Created March 16, 2016 and submitted
Integrate with Wayland and create a ShadowManager if available. With the ShadowManager it's possible to create a Shadow for a Surface. The Wayland shadow is very similar to the X11 based one, so a lot of code can be shared. The code is slightly refactored to share the common code paths to create and destroy the shadow.