* during drag and trim operations, we use "dragging region" transparency
so you can see the audio data beneath your region(s)
* we already do this for region-end-trim fade drags, which is a special
kind of trim operation, accessed from the 'lower' drag handle
* now we also trigger transparency for regular fade trims.
see discussion at:
https://discourse.ardour.org/t/regions-transparency-in-7-2/108060/31
Previously that caused an assert() in audio_clock.cc:1964
current_duration(): Assertion `is_duration' failed.
Optimized builds always returned zero duration.
* HitCreateDrag::start_grab does not need to do anything
a hit cannot be past end of region. Drawing at or past
the end of a region will create a new region (not an event)
* HitCreateDrag::finished now calculates the position like
NoteCreateDrag::finished.
* the selection does not need to be cleared. Creating new
notes/hits selectes the newly created ones.
The old code could not snap to the grid, because it had a lot of confusion about pixels vs. time,
and between line-origin-relative time and absolute time
Despite what the docs say Gtk::HScale(0,1,s) with a step-size
s > 0 has a range [0, 1 - s]. GTKMM does allow for a step-size
of zero, which also works around this issue.
This works because gtkmm sidesteps gtk_hscale_new_with_range() which
would fail with g_return_val_if_fail (step != 0.0, NULL);
The reason for this is that gtkmm creates an Adjustment with a
page-size = step-size:
```
Adjustment* adjustment = manage(new Adjustment(min, min, max, step, 10 * step, step));
```
and `gtk_adjustment_configure` limits the range:
```
value = MIN (value, upper - page_size);
```
