Added multiple track export feature.

Here is the patchset for my multiple track export feature. It works now
the way I originally envisioned.

For example, after I export tracks on my little song, I see this in the
directory I created:

devin@devin-studio:~/lmms/projects/fff$ ls
0_Defaultpreset.wav  3_Defaultpreset.wav      6_csidSouwav.wav
1_Defaultpreset.wav  4_spacenoiseswavwav.wav  7_HHOPENwav.wav
2_Defaultpreset.wav  5_csidkickwav.wav        8_HHOPENwav.wav

Each instrument or sample track is exported individually, regardless of
whether in its own song track or playing as part of a BB track. The
name is taken from either the song track name or from the BB track name.
My goal was to get the tracks individually exported, so that I could
combine them with other tracks in Ardour.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f