Support individual buttons for SRAM AXS

[jonasbark]

I looked into it and the functionality is "simple":

d9050054-90aa-4c7c-b036-1e01fb8eb7ee is notifying about the click and then you read the characteristic data from the same characteristic to determine which button was pressed - and there's the issue. It seems to use encryption, so I'm unable to analyze it.

So for now, I'll implement it like this. Better than nothing, but not great:

All buttons are mapped to the same action, but you can have two actions by performing a single click or performing two fast clicks and then assigning an action to each in the app settings.

Default:
Single Click: Shift Up
Double Click: Shift Down

Originally posted by @jonasbark in #152