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 allows UI scripts (saved in preferences) to access
os.* functions (non-sandboxed), while preventing other
scripts to do so.
Lua scripts that can run os commands can execute arbitrary
code on the system. While this is a nice feature, it can be
equally dangerous.
Without this, Editor::button_release_handler() will handle a button3 press
without checking for an active drag, resulting in two drags, and two reversible
commands, and an abort ...
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.
