The box outlines a given region, the grab position may be earlier
than the region's position.
Previously this caused a crash (uncaught exception):
```gdb
__cxa_throw () at /lib/x86_64-linux-gnu/libstdc++.so.6
Temporal::timecnt_t::timecnt_t(Temporal::timecnt_t const&, Temporal::timepos_t const&) at ../libs/temporal/timeline.cc:79
RegionMoveDrag::setup_pointer_offset() at ../gtk2_ardour/editor_drag.cc:2226
```
heap-use-after-free. Marker is deleted (and re-created), when
the tempo-map edit is aborted:
```
#0 0x7f77528ac017 in operator delete(void*) ../../../../src/libsanitizer/asan/asan_new_delete.cpp:160
#1 0x55f81062800d in TempoMarker::~TempoMarker() ../gtk2_ardour/marker.cc:794
#2 0x55f80ffb2fc0 in Editor::reset_tempo_marks() ../gtk2_ardour/editor_tempodisplay.cc:205
#3 0x55f80ffb2b19 in Editor::reset_metric_marks() ../gtk2_ardour/editor_tempodisplay.cc:185
#4 0x55f80ffb49fb in Editor::tempo_map_changed() ../gtk2_ardour/editor_tempodisplay.cc:301
#5 0x55f80ffbdf00 in Editor::abort_tempo_map_edit() ../gtk2_ardour/editor_tempodisplay.cc:850
#6 0x55f80fcf967a in TempoMarkerDrag::finished(_GdkEvent*, bool) ../gtk2_ardour/editor_drag.cc:333
```
Since no movement occurred, the tempo-map was not changed.
however we need to drop the lock and writable thread-pointer...
This involved a significant change in event handling for automation region views,
but it brings it into line with how it works for other things. On button press
we initiate a drag, then if no motion occurs, the Drag returns false during
finalization, and only then do we continue through Editor::button_release_handler()
to eventually end up in ::add_automation_event().
Although it is a substantial change, the fact that it now works the same
way for audio regions, automation regions and automation tracks seems
like a definite plus.
Drag::_raw_grab_time is set to the correct time-domain. However
during ::motion() and other operations the adjusted time was
always AudioTime.
Most notably this caused editor->selection->time to have
different time-domains for start and end.
This prevents overlap of successive hits.
Ardour 7 added a new duration select drop-down menu,
which defaults to "Auto" (musical grid) and removed
the special case when drawing percussive notes.
It was possible to accidentally create overlapping notes,
without the user being aware of doing so.
