Hold chord to hold down modifier

[user202729]

(currently just an idea. I don't need this very frequently. If I end up implement this I might post in this discussion)

Occasionally, I would need to, for example, hold down Ctrl while drag and drop with the mouse.

Currently this is not possible to do with the steno keyboard.

With the new locking keys feature (#1278) it's possible, but is quite cumbersome -- you need to remember to press another chord to release it)

Idea:

• When a chord (A) is held down for time[A] milliseconds, let the machine plugin send the chord down_chord[A].
• Then, after initial_delay_time[A] milliseconds, if the chord is still held, repeatedly send a chord hold_chord[A].
• When the chord is released, or if more keys is pressed (with or without first-up chord send), send the chord up_chord[A].
• The configuration is stored somewhere.

Of course this is not possible with some machines (keyboards) that use serial protocol, but most such machines
can emulate a NKRO keyboard just as well.