Implement basic battery manager.

[kareltucek]

Solves the most pressing issues of #974.

What it does:

• implement controls that allow easily setting battery limits
• implement a state machine that enables and disables the charger as needed
• implement a charging mode for stationary use that targets 3.8V, and its macro controls
• eliminate the big step of battery indicators between powered and unpowered states by:
  • when measuring the voltage, disable charging and allow the battery to settle for 500ms before measuring
  • apply a simple correction to the value as 500ms is not sufficient for the voltage to completely stabilize

What it does not do:

• adaptively detect 100% voltage
• it does not apply any artificial smoothing of the percentage
• does not apply logarithmic correction to the battery percentage

[kareltucek]

This isn't tested very well yet.

[kareltucek]

Known issues:

• battery presence detection is broken. fixed
• there is still the step caused by whether battery is being drained or not during measurement. (I don't plan to fix this at the moment.)
• i yet plan to implement shutdown mode when battery drops bellow "0". (It looks really bad when battery indicator says 0, but the keyboard still runs happily. implemented, although in a very basic manner.

[mondalaci]

The code looks very good and well abstracted.

If the charger is disconnected, what policy will you implement for ChargeRegion_Empty vs. ChargeRegion_AlmostEmpty? It might be worth displaying Charge me on the OLED for either or both and not enabling per-key backlight in the first place.

[kareltucek]

For empty, uhk shuts down (as deep as I can make it).

Almost empty is just an auxiliarry region that makes sure that the keyboard doesnt oscillate between power on and off.

I am not displaying anything at the moment.

[mondalaci]

Sounds reasonable.

Charge me may make sense for ChargeRegion_AlmostEmpty, but we'll see what works in practice.

[kareltucek]

I am not strictly against introducing the warning, but I would prefer to split into a separate issue (I am already writing another code on top of this, am not sure where to render it, and am not even sure we need it)...

[kareltucek]

I think this is ready for merge.

[kareltucek]

Actually, not yet. Half will not wake up if restarted 🤦.