Add an Arc tool and a handle to pull a half arc in or out (#408)

The diagram toolbar gets an "Ajouter un arc" button next to the
ellipse. Click one end, then click the other end at the height the arc
should reach: the result is a half arc bulging up or down from the line
between the two clicks (Shift: a true half circle). It is not filled,
since a fill would close it into a half disc.

A selected half arc shows one more handle, in the middle of the curve.
Dragging it makes the arc deeper or flatter while both ends stay put;
dragging it across the line between the ends turns the arc over. The
handle is hidden whenever the arc is not a half arc on a horizontal or
vertical diameter, because only then is "keep the ends, move the
middle" one well-defined change.

Nothing new is stored: an arc stays an Ellipse with a start and end
angle, exactly as the existing arc handles of an ellipse save it, so
files read by older versions are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/diagramevent/diagrameventaddshape.cpp

@@ -34,10 +34,14 @@
 	Default constructor
 	@param diagram : the diagram where this event must operate
 	@param shape_type : the type of shape to draw
+	@param half_arc : with shape_type Ellipse, draw a half arc instead of
+	a whole ellipse (the Arc tool). It is still an Ellipse with a start
+	and end angle, the same shape the arc handles of an ellipse produce.
 */
-DiagramEventAddShape::DiagramEventAddShape(Diagram *diagram, QetShapeItem::ShapeType shape_type) :
+DiagramEventAddShape::DiagramEventAddShape(Diagram *diagram, QetShapeItem::ShapeType shape_type, bool half_arc) :
 	DiagramEventInterface(diagram),
 	m_shape_type (shape_type),
+	m_half_arc (half_arc && shape_type == QetShapeItem::Ellipse),
 	m_shape_item (nullptr),
 	m_help_horiz (nullptr),
 	m_help_verti (nullptr)
@@ -131,6 +135,23 @@ void DiagramEventAddShape::applyPosition(const QPointF &pos, Qt::KeyboardModifie
 		return;
 	}
 
+	if (m_half_arc)
+	{
+		// The first click is one end of the arc and sets the height of
+		// its chord; the cursor sets the other end and how far the arc
+		// bulges, up or down, from that chord. Shift makes it a true
+		// half circle.
+		const qreal width = qAbs(pos.x() - m_anchor_point.x());
+		qreal height = pos.y() - m_anchor_point.y();
+		if (mods & Qt::ShiftModifier)
+			height = (height < 0 ? -width : width) / 2;
+		const qreal depth = qAbs(height);
+		m_shape_item->setRect(QRectF(qMin(pos.x(), m_anchor_point.x()), m_anchor_point.y() - depth,
+									 width, 2 * depth));
+		m_shape_item->setEndAngle(height <= 0 ? 180 : -180);
+		return;
+	}
+
 	m_center_anchored = (mods & Qt::ControlModifier)
 			&& (m_shape_type == QetShapeItem::Rectangle || m_shape_type == QetShapeItem::Ellipse);
 	if (m_center_anchored)
@@ -227,15 +248,20 @@ void DiagramEventAddShape::mousePressEvent(QGraphicsSceneMouseEvent *event)
 			// live on every subsequent move, from whatever Ctrl state is
 			// held at the time.
 			m_center_anchored = (event->modifiers() & Qt::ControlModifier)
+					&& !m_half_arc
 					&& (m_shape_type == QetShapeItem::Rectangle || m_shape_type == QetShapeItem::Ellipse);
 			if (m_center_anchored)
 				showCenterMarker(m_anchor_point);
+			if (m_half_arc)
+				m_shape_item->setEndAngle(180);
 				//Start from whatever pen/brush was last applied this
 				//session, rather than always the hardcoded default.
+				//Not the brush for an arc: it is a line, and a fill
+				//would close it into a half disc.
 			if (LastUsedStyle::hasShapePen()) {
 				m_shape_item->setPen(LastUsedStyle::shapePen());
 			}
-			if (LastUsedStyle::hasShapeBrush()) {
+			if (LastUsedStyle::hasShapeBrush() && !m_half_arc) {
 				m_shape_item->setBrush(LastUsedStyle::shapeBrush());
 			}
 			m_diagram->addItem (m_shape_item);
@@ -461,6 +487,8 @@ QString DiagramEventAddShape::beforeClickHint() const
 			return tr("Clic gauche : positionner le point de départ (Ctrl = position libre)");
 		case QetShapeItem::Rectangle:
 		case QetShapeItem::Ellipse:
+			if (m_half_arc)
+				return tr("Clic gauche : positionner une extrémité de l'arc (Ctrl = position libre)");
 			return tr("Clic gauche : positionner le premier coin (Ctrl = point central, position libre)");
 		case QetShapeItem::Polygon:
 			return tr("Clic gauche : positionner le premier point (Ctrl = position libre)");
@@ -479,6 +507,9 @@ QString DiagramEventAddShape::afterClickHint() const
 			return tr("Clic gauche : positionner le coin opposé (Maj = carré, "
 					"Ctrl = depuis le centre + position libre, Ctrl+Maj = carré centré) ; clic droit : annuler");
 		case QetShapeItem::Ellipse:
+			if (m_half_arc)
+				return tr("Clic gauche : positionner l'autre extrémité et la hauteur de l'arc "
+						"(Maj = demi-cercle, Ctrl = position libre) ; clic droit : annuler");
 			return tr("Clic gauche : positionner le coin opposé (Maj = cercle, "
 					"Ctrl = depuis le centre + position libre, Ctrl+Maj = cercle centré) ; clic droit : annuler");
 		case QetShapeItem::Polygon: