Add grid filter to rhythm ferret and grouped transient lines

[Houston4444]

The idea is to add a simple checkbutton in rhythm ferret window "filter on grid". At the end of analyze, it just retains the result closest from the grid bar if there are many for the same bar.

With experience of that, I decided to change behaviour of transient feature lines in equivalent regions,
when user drags a transient line in a region, it also drags all equivalent transients (transients at same position on equivalent regions). This gives a really nice comparaison to audio waveform.

To move only one transient line, just press primary key, as Ardour does for many things.