Implements count-in, playing 'tick-tack' each time playback is started.

Ticks are always played for at least one full measure (nominal time signature at playback starting point). If the playback starting point is not at the beginning of a measure or if the measure is shorter than nominal (anacrusis), also plays enough 'tick-tack' to fill the measure.

Count-in is turned on / off with a button in the play panel.

A provisional button icon is supplied (roughly representing a conductor hand and stick); a better icon is probably needed.