Proposals/Support for multiple tags

Summary

Add Support for Multiple Artist/Album/Genre/... Tags to Amarok

ID3v2 and many other Tagging Systems allow Multiple Tags. Amarok ignores them. Users wish for mutliple Tags to be read & to be editable by Amarok.

Please discuss this proposal on its discussion page.

) Try how TagLib Support looks - done, example file see tagtest.cpp Gist
) Plan how to save multiple Tags in the XML File of the Collection Scanner - planning example: http://piratepad.net/QYDQn3Ardp
) Change Collection Scanner to scan all the Multiple Tags and put them in the XML File
) Plan how to save them in the Database
) Change MetaTrack to return lists for genre and others. So instead of a GenrePtr genre() const; we should have GenreList genres() const;
) Change Database. We would have a standard normalized database table layout. e.g. since Tracks<->Genre is a n:m relation we need a relation table. (change DatabaseUpdater)
) Change Classes (SqlMeta, SqlRegistry and SqlRegistryPrivate) to handle the changed database layout.
) Change the ScanManager to use all the new stuff.
) Plan how to display multiple Tags in the UI
) Change UI (CollectionBrowser, CurrentTrack Applet, Meta Data editor, Playlist items)

) Start after 2.4.2 release
) Use independent changes. E.g. changing the collection scanner XML but for a first step not using the information. Or returning GenreLists but for a first try just using the first entry, disregarding further genre results. This working mode allows us to always have a running Amarok and no need for a big merge when everything is done.
) Work on a separate branch when we need to put all the parts together.