[Feature enhancement] xpgen and xigen for more detailed generators

[sylvia-leaf]

Is your feature enhancement request related to a problem? Please describe.
Right now, generator values are limited to two bytes, as in SF2.04.

SFE 4.0.31 (and SF2.04) defines the generator formats as:

• two uint8_t
• one int16_t
• one uint16_t

This is because the current xdta-list specification does not specify the use of pgen and igen.

Describe the solution you'd like
Define what pgen and igen mean as part of the xdta-list sub-chunk.

This can differ based on the generator format, and can provide things such as increased parameter precision, effect subtypes, and advanced parameters.

This would also be especially useful if it allows players to implement popular extended GM standards.

It would also allow structural design: for example, if there is something that we only want users to select one of, but can be adjusted, then we could use xdta-list to allow this to happen.

Describe alternatives you've considered
Because up to 65,536 generator enums can theoretically be defined, we could use them like that, but the structuring improvements that are possible by using xpgen and xigen is sufficiently beneficial and the infrastructure for such a feature is already there.

Additional context
According to my research, it appears that the XG message format allows up to 16,384 insertion effects, while each of the SC insertion EFX subsystems has 64 different types of effects, but with a possibly higher theoretical number for SC.

[stgiga]

I like this idea!