Refactor Toplevel::opacity

The major difference between this version and the previous is that kwin
no longer forwards the opacity to the app window if it runs as a
window 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