Make sure at least one position change event is emitted after

Make sure at least one position change event is emitted after
a timeline drag (with frequent=false), even if lots have been sent
with frequent=true.

Otherwise the code in the CHNAGE_TYPE_DONE branch of Butler::player_change
never gets to seek (since frequent is true, it ignores the signal).
Without the seek things go wrong.

Believed to fix #1534.