Add GBA support

[akumanatt]

This adds 2 systems, one directly uses the DMA to read and play source samples, another one uses a software mixing driver called MinMod. The GB core is also modified to support GBA's wave channel extension.

Like QuadTone, I already have a proof-of-concept mixer code implemented (I got CPU cycles estimate this way):

zqotPknSTV_.mp4

MinMod echo notes: This works differently from SNES and QSound where you need a free channel to play it since under the hood it's just reusing the mix buffer as a source sample. Simply play a very high note with 11xy effect (the pitch will be clamped to sample rate).

TODO:

• System IDs (yes, please)
• Instrument icons (done in Furnace-B already but I don't have tools to edit it)
• Double-length wave flag save/load in GB instrument

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Chip IDs allocated.
Use 0xd7 for direct and 0xd8 for MinMod.

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Instrument icons

Now done. ICON_FUR_INS_GBA_DMA and ICON_FUR_INS_GBA_MINMOD

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Further work may be continued in Git master - feel free to do it