improve mouse zooming for Qt plotting (bug #44302) * Figure.h (enum MouseMode): Split ZoomMode into ZoomInMode and ZoomOutMode. Change all uses. * Canvas.cc (Canvas::canvasMouseReleaseEvent, Canvas::canvasWheelEvent): Make meaning of zoom factor consistent with zoom function. Make direction of wheel event consistent with other programs. * __init_fltk__.cc (plot_window::handle): Make meaning of zoom factor consistent with zoom function. * MouseModeActionGroup.cc (MouseModeActionGroup::MouseModeActionGroup): Provide buttons for zooming in and out. * graphics.cc (figure::properties::set_toolkit): Handle zoom direction. (do_zoom): Make factor > 1 zoom in.

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