Mpris CanPlay=true never gets signalled

VLC never sends the signal that CanPlay = true on DBus, so MPRIS clients (e.g. KDE Connect) don't know that playing is allowed. This results on a broken UI after pausing, since the play button is disabled.

After a bit of debugging, I think this is what happens:

VLC starts with an empty playlist (because it's populated asynchronoously?)
VLC becomes available on DBus, gets queries, and correctly replies with CanPlay = false
The playlist becomes available, but CanPlay = true is never sent

After looking a bit at the VLC code, it seems CanPlay = true/false gets signalled after playlist additions/deletions. Testing confirms that this works as intended: after deletion/addition/clearing CanPlay = true is sent. So I think the only thing missing is an playlist updated call when it's finished loading initially (but I can't find where to do that).

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