double-tap fn1 to lock

[jonhiggs]

It's annoying to use the fn1- combo with one hand.

Also add an LED that is lit when function is locked on.

[jonhiggs]

it might be better to move fn2 to bottom left, then replace the existing fn2 to a locking fn1.
That will allow a single-handed locking of vim arrows.

[jonhiggs]

haven't wished for it so I must not need it.

[jonhiggs]

I want this again.