Mtp: show tracks which are in subfolders from Music/ (bgo#724678)

Files can happen to be in a hierarchy of folders inside the
Music/ folder in the device. To be able to see them we need
to do add them the proper MtpTrackInfo.MediaAttriubutes by
looking for them recursively.

Based on original patch by IBBoard <dev@ibboard.co.uk> (his
patch included read-and-write support, this patch only
implements read support for now. Write support will need
more discussion. See BGO#539804 for the latter.)