Add helpers to support track folders

[CharlesHolbrow]

Tracks are all stored at the same level in Reaper.

A simple test reveals three types of ISBUS structs within a <TRACK ...> object

• ISBUS 0 0 indicates a track that is neither a folder OR an folder-end
• ISBUS 1 1 indicates a track that has children, and the subsequent track will be "one level down"
• ISBUS 2 -1 indicates the last sub-track within a folder. The second number indicates how many levels up the subsequent track is, for example...
• ISBUS 2 -2 indicates a closing track, and that the subsequent track will be two levels "up" from this track

So

• The first number is modal 0=normal, 1=folder, 2="folder end"
• The second number is indicates which level the subsequent track will be (relative to this track)