Button long-press alternate function

[charlespax]

Each button could have an alternate function. Enter this mode by giving a long press.

Ideas:

• Graph button - Show large digits https://github.com/PaxInstruments/t400-firmware/issues/48
• Graph button - Show mix, max, avg, etc. https://github.com/PaxInstruments/t400-firmware/issues/199
• Backlight button - Adjust contrast
• Backlight button - Adjust backlight intensity (it's on a PWM pin) https://github.com/PaxInstruments/t400-firmware/issues/142
• Timebase button - Adjust date/time https://github.com/PaxInstruments/t400-firmware/issues/153
• Temperature units button - Select other thermocouple types