Delay releasing the compositor selection

When turning off the compositor do not release the compositor selection
directly but delay it through a timer. The idea is that the internal
change when e.g. restarting the compositor or switching from XRender to
OpenGL should not be visible to the outside world.

This hopefully makes restarting the compositor more robust in Plasma due
to the SelectionWatcher sometimes reporting incorrect results.

When restarting KWin the change does not matter as the selection gets
force claimed by the new instance anyway.

CCBUG: 179042
REVIEW: 106844