- Don't do anything if no MPV created
- By default don't create the MPV, let the user do it himself

MPV: Don't load the video by default, force the user to click on the button to avoid some strange race conditions

The race condition between Qt's widget creation and mpv getting the WID
doesn't seem to be fixed for now, will need to investigate further.

MPV: Use mpv_destroy to delete the player, don't use the "quit" command as it seems to be the way recommended in the client.h header

Try to force the window creation before passing the WID to MPV to fix
the possibility where the window is not already created and thus
everything crashes.