Adding manual harmonics sometimes not vis when zoomed in #130
Description
When a set of harmonics are added manually, they are placed in the centre of the time period.
But, when the view is zoomed in, the centre of the time period may not be visible - so the new set of harmonics may not be visible until the user zooms out.
The user may have zoomed in on a feature in the spectrogram to align harmonics against it - but then has to zoom out to see it.
The base time for the new set of harmonics should be the centre of the visible time period, not the overall time period.