This is an improvement on retro-tap, solving the 'flashing hold' issue
users people experience.
When the tapping-term expires, the hold key is normally pressed. When
retro-tap is enabled, this is undesirable; only an interrupted hold-tap
should trigger the hold behavior.
This change disables the hold behavior for the 'STATUS_HOLD_TIMER'
state when retro-tap is enabled, and makes sure the
'STATUS_HOLD_INTERRUPT' state will be triggered when appropriate.
Tap-and-hold a hold-tap to hold the tap behavior so it can repeat.
After a tap, if the same key is pressed within `quick_tap_ms`, the
tap behavior is always picked.
This is useful for things like `&ht LSHFT BACKSPACE` where holding
the backspace is required.
Implements #288.
