Rework blocking compositing on X11

Currently, the Workspace is responsible for rerouting
X11Window::blockingCompositingChanged to
X11Compositor::updateClientCompositingBlocking(). It has a few issues:
if the client is initially blocking compositing, it's not going to work
as expected. The second issue is that it creates a coupling between
platform specific compositor implementation and generic Workspace. It's
a blocker for moving X11Compositor to kwin_x11 executable, etc.