MIDI Constant Defines - Firmware & Config Tool

[MrMep]

Second and last PR addressing the definition of constant values #5 After the first one about EEPROM addresses, this one is about MIDI numerical values.

I understand that it's hundreds of changes, it will require some time to be validated and maybe some changes might look like a futile exercise, but I tried to be consistent and achieve multiple goals:

1. Avoid numerical repetitions in the code (which are frequent cause for errors)
2. Define human readable constants thus making the code easier to understand
3. Making the code easier to maintain and extend.
4. Having an in-code map of used (and available) cc values.
5. Having a consistent naming across firmware and Config tool.

A few times I found an inconsistency between what indicated in the Google Doc and in the code: these are marked in comments beginning with "MrMep": feel free to remove them.

I think I found a small problem, documented #8

I hope I haven't made mistakes but, hey, I swear I'm human.

Brief Changelog Firmware:

• Added constants defines for MIDI numerical values
• Substituted (almost) all MIDI numerical references with constants defines

Web Config Tool:

• Added a new file constants.js
• Modified HTML configure pages to include the new constants.js
• Substituted (almost) all MIDI numerical references with constants defines

[amowry]

Thanks!! I'll start looking at this on Monday.