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

Ticks are always played for at least one full measure. 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.
Merge pull request #504 from mgavioli/count_in

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