Invaild MPRIS trackId's

[JasonLG1979]

System information

Please provide information about your system and the version of Clementine used.

• Operating System: Fedora 26
• Clementine version: 1.3.1

Expected behaviour / actual behaviour

Expected: TrackId's should start with a reverse DNS of the player and should be unique up to the point that if you repeat a track and/or it appears multiple time in the TrackList interface it should have a different TrackId .
Example: Pithos TrackId's start with '/io/github/Pithos/TrackId/'

How you make them unique is up to you so long as that if a client uses a TrackId in a method you know what track that TrackId is referring to.

Actual: Clementine TrackId's start with '/org/mpris/MediaPlayer2/Track/' which is forbidden by spec unless you are referring to "NoTrack".

https://specifications.freedesktop.org/mpris-spec/latest/Player_Interface.html#Simple-Type:Track_Id

Even if/when you fix #5794 your TrackList interface will still be broken until this is also fixed.

Steps to reproduce the problem (only for bugs)

Read the metadata of a track.

https://github.com/JasonLG1979/gnome-shell-extensions-mediaplayer/wiki/Known-Player-Bugs#clementine

[JasonLG1979]

Clementine/src/core/mpris2.cpp

Line 323 in e5ab3e7

return QString("/org/mpris/MediaPlayer2/Track/%1")