Close the media panel to really close the application

Dear Martin,

There is little issue with the media panel that can often prevent Joker from exiting properly. I think you mentioned that in a previous PR comment. The thing is that closing Joker main window does not close systematically the media panel, so that it can stay opened in the background but hidden. This PR fixes that by closing the panel when the main window is closed.

Best regards,

Timothée