Viewport: scroll horizontally with the vertical wheel and the shift key

Some software (notably Gimp and Inkscape) allows horizontal scrolling
with the vertical wheel when the shift key is pressed. Implement this
behaviour for easier navigation with single wheel mice.

pv/views/trace/viewport.cpp

@@ -241,6 +241,12 @@ void Viewport::wheelEvent(QWheelEvent *event)
 			// is intrepretted as vertical scrolling
 			view_.set_v_offset(-view_.owner_visual_v_offset() -
 				(delta * height()) / (8 * 120));
+		} else if (event->modifiers() & Qt::ShiftModifier) {
+			// Vertical scrolling with the shift key pressed
+			// acts as horizontal scrolling like in Gimp
+			// and Inkscape.
+			view_.set_scale_offset(view_.scale(),
+				- delta * view_.scale() + view_.offset());
 		} else {
 			// Vertical scrolling is interpreted as zooming in/out
 #if QT_VERSION >= QT_VERSION_CHECK(5, 12, 0)