Refactor some X11 opacity bits

The major difference between this version and the previous is that kwin no longer forwards the opacity to the frame window if it runs as a window manager and a compositing manager. As Keith Packard said [1]

The window manager support is only needed to forward the property from the application window to the frame; with the Composite extension, a compositing manager can then take that value into account when constructing the desktop image. Any X server can implement the Composite extension; the one in the X server project at freedesktop.org is just one such. It would be trivial to implement this extension in a direct FB based X server, or any other X server which places windows in off-screen images.

There are a couple of reasons to do so: (a) simplifies code, (b) we don't temper with opacities on override-redirect windows.

[1] https://listman.redhat.com/archives/xdg-list/2003-December/msg00153.html

Depends on frameworks/kwindowsystem!13 (merged)