Improve MainMenu, add brightness control

[TokenRat]

Improve MainMenu, add brightness control

Somehow it is not yet correctly persisted. I think I am loading globals incorrectly in main.cpp and that makes it reset this to the default value.

[TokenRat]

Based on #24 , merge this one first.

[ngandrass]

There is a 15-character limit for keys in Preferences / NVS. This most likely causes problems.

Namespace and key names are character strings and are limited to a maximum of 15 characters.

See: https://docs.espressif.com/projects/arduino-esp32/en/latest/tutorials/preferences.html#preferences-attributes

[ngandrass]

I pushed a potential fix but cannot test it right now.

@TokenRat Please test, confirm its working properly, and merge if you like 👍

[ngandrass]

Ok, there were more problems. A second FSMGlobals instance was created and the brightness was tried to be set before the values were read from the NVS. Now it should work and I did a super quick test on hardware.

Please test yourself and merge afterwards ❤️