Make MIDI controller's buttons assignable

[artur-twardowski]

Adds a possibility to assign keys or buttons to automatable controls.

While some MIDI keyboards allow assigning push buttons to CCs, other MIDI devices such as AKAI Midimix trigger Note On and Note Off messages. In current implementation it is not possible to link those buttons with on-screen controls. One can use a slider or a knob instead, but a physical push button is much more relevant for on-off switches, especially when working with plugins such as OpulenZ that exposes 11 "LEDs".

The change proposed adds two modes of operation when dealing with Note On and Note Off messages:

• each consecutive Note On message behaves as CC toggling between extreme values (0 and 127)
• each Note On message behaves as CC set to value of 127, each Note Off acts as CC reset to 0

Change in scope of #1472 (proposed by me in a comment), it also addresses #1815.

Note: I had to remove the infamous octave-shifting code from MIDI clients, however I see that some proper solution is now being worked on in #5349. Maybe the aforementioned change needs to be merged first, then this PR should be rebased on top of it.

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Linux

• Linux (AppImage): lmms-1.2.1.580-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.2.1.580-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.1.580-mingw-win64.exe (build link)
• Windows 32-bit: lmms-1.2.1-msvc2017-win32.exe (build link)
• Windows 64-bit: lmms-1.2.1-msvc2017-win64.exe (build link)

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