The second design from 2017, based on a pipeline builder,
is now renamed `TreeExplorer` ⟼ `IterExplorer` and uses
the memorable entrance point `lib::explore(<seq>)`
✔
some bugfixes,
but also a notable change: detect the completion of the gesture
directly when the button is released; this is necessary, because
seemingly we do not get motion_events when no button is pressed,
at least not in this test setup based on a Gtk::Button widget.
- most notably the NOBUG logging flags have been renamed now
- but for the configuration, I'll stick to "GUI" for now,
since "Stage" would be bewildering for an occasional user
- in a similar vein, most documentation continues to refer to the GUI
