Piano-roll record countdown

Kind of related to #475, because the metronome is involved.
When user want to 'record' notes during playback in piano-roll, a signal pre-sequence should be an optional choice ie:
di di di dyyy - att dyyy, the record _starts_
This gives the user a chance to be ready to follow the playback score.
I think it should be optional, because some would perhaps find it anoying (a simple toggle in setting)
A different aproach could be to use the ms counter, and let user insert a value, that then would count down as a visuel, but an audiel 3+1 secd sequence would do fine.
Init-event could be _right_-click, on either record-options in P-R

Start by locating the piano-roll record controls and the metronome implementation, using issue #475 for related context. Clarify whether the countdown is an optional setting or a configurable visual timer, and define the right-click trigger and expected behavior before implementation.

Written by the indexing model from the issue text.