qt: prevent UI's potential obstruction of video window

Currently, the video window gets already ready within window enable. However, since UI is an overlay, it blocks the video, potentially letting frames to not be displayed to the user.

In other words, the frames are rendered and available in the video window, but if the UI obstructs the video window (which is the case), we need to wait for UI to prepare itself for video display. This can currently only be done by blocking the decoder thread that calls window enable.

Request review @tguillem.