[ui] button_group: public state-bitfields and some refactor

[TomSaw]

Key intent here is to declare the state bitfields (pressed, released, repeated) public, so i can pass them around for remote interpretation.

By the chance, i gave some service: It's more compact and should be easier to read.
Interface is 100% like before except the state-getters returning bool, not T now - Guess that was a mistake.

Further Ideas but with API change:

• Type N-Buttons as template argument template<std::size_t N> improves generics: you can easily take the N from your enum ButtonIdentifier.

enum ButtonIdentifier {
  BUTTON1,
  BUTTON2,
  N
};

• move Button-data State pressed, State released and State repeated into external struct like it's for modernized modm:driver:**. So it's even easier to pass the states around and follows the convention.

[salkinium]

Looks fine!

[TomSaw]

Done. Have moved the whole docs into button.md

[salkinium]

Also something with the update() method broke the unit tests…

[TomSaw]

I've moved internal members completely into private scope. Only the ButtonGroup::State types kept being public.

[salkinium]

Thanks!