Keep KWin responsive while loading all effects

Loading all effects during startup can take some time[1] and during
that time the screen is frozen as the loading blocks the compositor.

This change doesn't load effects directly but puts them into a queue.
The loading is controlled by a single-shooting QTimer with 0 msec
interval. Thus we get a firing compositing timer in between and can
ensure that a frame is rendered when needed and also react to X events
during the loading.

[1] On my high-end system the set of effects I use take about 200 msec
    to load.

Added some debug statements to see whether we get frames rendered between loading. Yes, works.