Configuration: Add versioning support for StreamingProfiles.xml
MAS: Add a GetArtwork() method to get the artwork from a mediaitem.

Also rewrite GetPathList() to use GetArtwork() and GetMediaItem(), so that all artwork types
are now supported for all mediatypes, if the plugin supports them.
WSS: Support -1 as artwork offset to get the preferred artwork
WSS: Support picking all artwork in order of preference with negative offsets

This means that -1 maps to the preferred artwork, -2 to the second-best, -3 to
the third, etc.
WebMP: Use preferred artwork (fixes #386)
MovingPictures: Prefer configured cover above other artwork
Merge branch 'update-streamingprofiles'
Merge branch 'preferred-artwork'