qt: fix video surface initialization and de-initialization ordering issues

I wanted to initialize video surface provider and window handler within commonSetupVoutWindow(), since it is where the video window is set and video surface provider and window handler only makes sense when there is a valid vout window. However, for some reason, it did not work so it is a TODO for now.

Currently, since there are a lot of signal connections, changes are reported even when there is no valid vout window. For example, CompositorVideo::onSurfacePositionChanged() may be called when there is no vout window or when it is already invalidated.

Request review @chub.