Don't handle FinishedTrack as Stopped

[Qluxzz]

Currently FinishedTrack and Stopped both returned "Stopped" leading mpris to report stopped after every track and then playing again when the new track starts.

Not sure if I am allowed to not return a value when we don't have a valid PlaybackState value. The docs for the PlaybackState states "When this property changes, the org.freedesktop.DBus.Properties.PropertiesChanged signal is emitted with the new value."

but I'm not very knowledgeable with DBus.

I've not encountered any weird behaviour using the same debugging technique described in #395.

Fixes #395

[hrkfdn]

Thanks for tackling this. I took a look to see what happens if we just add nothing with gdbus introspect --session --dest org.mpris.MediaPlayer2.ncspot --object-path '/org/mpris/MediaPlayer2' --only-properties and in those cases the value seems to be empty, which it seems would violate the MPRIS specs. I'd suggest to just keep the state at Playing when FinishedTrack was emitted, as it should be followed by a Stopped event. I will commit the changes in a second.