Add support for AdLib Gold banks

[Wohlstand]

It's another variant of AdLib's BNK bank format which stores 4-op instruments.

The working implementation from off GenAdldata: https://github.com/Wohlstand/libADLMIDI/blob/master/utils/gen_adldata/file_formats/load_bnk2.h

• Read/import
• Writing

[jpcima]

BNK2 takes a prefix to filter which instruments are going in the melodic or percussion bank.
How to handle this? can I pop up a dialog box?

[Wohlstand]

That filter was a workaround to "Master of Magic"'s bank, for now, let's just make the non-GM bank for debug purposes, and then look for the internals better...

[Wohlstand]

JPCima have made experimental read-only:
#98