[FEATURES REQUEST:] Add diagnostic mode to DDIs/AMPCD.

[jrsteensen]

DDIs:

On Pro-Micro Power On

1. All backlights fade from 0 to full and back to zero before syncing with DCS.
2. Upon sync with DCS, backlights flash three times.

Diagnostic Mode

1. Hold any three pushbuttons for three seconds to enter.
2. All backlights activate full bright.
3. As you push each pushbutton, rotate rotary switch, or move encoders, all backlights flash off and on.
4. Hold any three pushbuttons for three seconds to exit.

AMPCD

1. Similar functionality as DDI.