Add support for audio files with cue sheets

Submitted by Carlos Garnacho @carlosg

Link to original bug (#763304)

Description

gnome-music could be able to handle audio files with cue sheets, where songs themselves are portions of the larger file, with an specified start time and duration.

In this case tracker stores the info like this (very simplified):

urn:example:file1 a nfo:FileDataObject, nfo:Audio; nie:url "file:///example/album.flac" ; nie:isStoredAs urn:example:file1 ; nfo:duration 10000 .

urn:example:song1 a nmm:MusicPiece, nfo:Audio; nie:isStoredAs urn:example:file1 ; nfo:audioOffset 0; nfo:duration 1000 .

urn:example:song2 a nmm:MusicPiece, nfo:Audio; nie:isStoredAs urn:example:file1 ; nfo:audioOffset 1000; nfo:duration 3000 .

...

gnome-music can generically query for the normal case, and this indirect relationship (because in regular files nie:isStoredAs points to self), and the nfo:audioOffset/duration properties are enough to make it behave as expected with these songs.

I'm attaching a few patches.