use Qt signals for interpreter callbacks in Qt graphics toolkit * Backend.h, Backend.cc (Backend::m_interpreter): New data member. (Backend::createObject): Pass pointer to invoking Backend object in signal argument list. (Backend::interpreter_event): New slots. Access interpreter event_manager to post interpreter callbacks events here. * ObjectFactory.h, ObjectFactory.cc (ObjectFactory::createObject): New arg, backend. Connect interpreter_event signal from newly created object to interpreter_event slot in backend object. * __init_qt__.cc (__init__): Pass interpreter to Backend constructor. * module.mk (OCTAVE_GUI_GRAPHICS_MOC): New files, moc-Canvas.cc and moc-Container.cc. * Object.h (Object::interpreter_event): New signals. * Canvas.h, Canvas.cc (Canvas::interprter_event): New signals. (Canvas::canvasMouseReleaseEvent): Emit interpreter_event signal instead of accessing interpreter event_manager object directly here. * GLCanvas.cc (GLCanvas::do_print): Emit interpreter_event signal instead of accessing interpreter event_manager object directly here. * Container.h, Container.cc (Container::interpreter_event): New signals. (Container::canvas): Forward Canvas interpreter_event signals to Container interpreter_event signals. * ButtonGroup.cc (ButtonGroup::ButtonGroup) Forward Container interpreter_event signals to the ButtonGroup interpreter_event signal. Since ButtonGroup is derived from Object, this connection and the one made by ObjectFactory::createObject has the effect of passing the interpreter_event signal up to the Backend interpreter_event slot. * Figure.cc (Figure::Figure): Likewise, for Figure. * Panel.cc (Panel::Panel): Likewise, for Panel.

--- a/libgui/graphics/GLCanvas.cc	Mon Jul 29 10:43:04 2019 -0400
+++ b/libgui/graphics/GLCanvas.cc	Mon Jul 29 10:43:20 2019 -0400
@@ -174,10 +174,7 @@
           }
         catch (octave::execution_exception& e)
           {
-            octave::event_manager& evmgr
-              = octave::__get_event_manager__ ("GLCanvas::do_print");
-
-            evmgr.post_event
+            emit interpreter_event
               ([] (void)
                {
                  std::rethrow_exception (std::current_exception ());