Device actions

[mondalaci]

This is the firmware-side issue of UltimateHackingKeyboard/agent#2502

A new key_action_type_t value has to be added for special actions. For clarity, it'd make sense to add one id for connections and another for the other special actions, which are expected to be extended later, but if we're short on key ids (assuming some bits of this byte are used in the RAM for other purposes), feel free to use a single, shared key_action_type_t value.

It'd be nice to visualize the empty/nonempty connection slots by showing the empty ones with half the brightness of their color when functional backlighting is used. (I'll extend the default configuration to map the "switch to connection 1", "switch to connection 2", ... actions to neighboring keys of the Fn2 layers.)

The new functional key color for special key ids must also be added.

[mondalaci]

Let's call these actions device actions according to the related Agent issue.

I suggest using the first 20 values for host connection slot ids and for the rest of the values, we could start from 255 and keep decreasing. This way, should we ever want to extend the 20 connection slots, it'd be easy.