Control `Mode` with `watch` channel

[ddrcode]

State handling (Mode) is ugly. There is Modechange event that just informs the change and Metexed avutal value available through state. And some local states copying from it.

Proposal

• replace Mutex with watch channel for Mode
• now each interesting party can observe the change without dedicated Event
• completely clean usage of the event
• it will reduce KeyScanner and qmk to pure event producers as the only event they were listening to was ModeChange