Pattern and song tunings

[Harbinger]

Tunings are now very flexible and seem to be working fine. Now i'd like to request one more step for the .mptm file format:

Right now tunings are saved with Instruments when using .mptm. If possible i would like to see an .mptm song have a default tuning, and every SONG have a custom tuning.

Also, every PATTERN could have its own tuning in case the composition changes keys. This will make it easy for many classical and experimental pieces to be more easily tracked with MPT, as visual verification and editing is instant.