Since 80c11a763a the GUI is notified when the xrun counter
is reset (previously the rec-ui xrun counter never returned
to zero).
However when create-xrun-marker is enabled ARDOUR_UI::xrun_handler
tried to create a marker at -1 which causes various issues.
There are various ways to cancel a dialog. Only checking for
RESPONSE_CANCEL is not sufficient. e.g. Esc causes a delete-event.
* Gtk::RESPONSE_CLOSE
* Gtk::RESPONSE_REJECT
* Gtk::RESPONSE_DELETE_EVENT
* Gtk::RESPONSE_CANCEL
* Gtk::RESPONSE_NO
Among others this fixes "Clicking session > open,
then hitting ESC opens the currently selected folder and session"
