Clear keys with a middle click

[Glitchy-Sheep]

❓ Problem

Clearing one key takes three steps:

1. open the picker
2. switch to the Special tab
3. choose None.

Emptying a group of keys means repeating that for every key, and the picker reopens each time.

This hurts most when reworking a layer for new functionality.
It's a lot of actions just to clean everything up (it could be a better UX for this).

Middle click had no meaning anywhere in the app, so it was free to use for this.

✅ Fix (what have I done)

• Middle click on a key or an encoder slot writes None (KC_NO) to the device without opening the picker.
• Clicks made while an earlier write is still running are queued, so fast repeated clearing does not drop any of them.
• A click on a key that is already empty does nothing: no write and no undo step.
• Each clear is its own undo step, so Ctrl+Z brings the keys back one by one.
• Clearing reuses the existing key write path, so RMK boards get the same write with readback that the picker already uses.
• When two edits target the same key, the newest one wins. A clear cancels a picker result that has not been written yet, and a handed swap waiting for Ctrl release. A layer write (paste, fill, undo) cancels clears queued for that layer.
• The bottom hint bar shows Middle click to clear this key while a key with a binding is hovered, and the onboarding tour text mentions the gesture.

🎦 Demo

I tested it out on my K:04, there is how it looks like (before / after):