Patch by Thomas Lübking:

Changes:
1) support for "autosizes" (setting "size" attribute to <= 0) - i'd suggest to make this the default behaviour, but don't mind at all
2) support for custom string elements (i.e. if the "value" attribute does not refer to a valid tag) to allow stuff like
<Song>" by "<Artist>" on "<Album>

Also i think there was a bug in the padding handling (but i'm not sure) and i removed (i think) redundant QFont(MetricsF) actions (and use the option.font instead of QFont())

Thanks for the patch, Thomas! I should have run this by Nikolaj first (he's the author of the new playlist rendering stuff), but the patch looked good to me, so I went ahead.