Implement 4-quad ("256") mode...

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... This requires a little bit of refactoring for "2-page" mode, and the various device config files.

@miker2049; If you don't like the "midigrid_2page.lua" -> "mg_128.lua" rename, please don't hesitate to say so.

My rationale for that rename was a global idea of continuing from here, to split out an "mg64.lua" from "midigrid.lua", and eventually refactor "midigrid.lua" to contain as much duplicated code from the "mg" as possible (note that I'm not* promising to do this 😝).

Notes:

• Important: This PR already includes most of the work from PR #6 and #5, so really they need to go in first.
• Tested "midigrid.lua", "mg_128.lua", and "mg_256.lua" with awake, loom, and tutorial/9_grid.lua
• Contributors who have access to the various devices which I don't (@jaggednz, @timm052, @ryanlaws), please add the new lower quad key note values for your device(s) to the relevant config file(s), otherwise 256 mode will only be 1/2 done 😁

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@jaggednz; thx, will fix and push as soon as I can. If you're comfortable enough with git/github, you're more than welcome to do so yourself; I think I set the magics properly so that others can push to this PR...

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@timm052, @miker2049; Now it's ready for merge 😁