When all players are closed, media player applet on the
desktop still shows a track from the last player, and
player control buttons are still clickable but have no effect.

Add a new signal named `playerListEmptied()`. When the proxy list is empty,
the signal is emitted and `m_multiplexer` is scheduled to be deleted, so
the media player applet on the desktop can indicate the right status of
no player opened.

BUG: 446098
FIXED-IN: 5.24