sleep mode: stop continuous blinking s+f and j+l keys during transport

[mhantsch]

When I package the UHK80 for transport, I put it to sleep mode. Pressing a key will flash the s+f keys (left half) or j+l (right half). That's OK.

However, if the key is continuously depressed, then the keys will continue flashing forever. So, if the keyboard is somehow squashed during transport, e.g. in some soft packaging material, and some keys are depressed, it will continue to flash the keys.

I suggest flashing the keys only for a few seconds after the press of a key, but stop flashing if the key is kept held down.

[mhantsch]

See https://photos.app.goo.gl/RNaB9m8EMk3z956u7 for an example of a wrapped up UHK80 that would continue flashing if squashed in its soft packaging during transport.