Fix saving of strokes that were partially erased

This ensures that when part of a line is erased,
`UBGraphicsStroke::isNominalLine` will no longer return true. This was
problematic as `UBSvgSubsetAdaptor` uses this to know whether or not to
save a stroke as polyline.
If it saves the stroke as a polyline, then the erased portions of the stroke
reappear after saving.

Therefore, we now force saving of the stroke as a group of polygons when it
has been partially erased.

src/domain/UBGraphicsPolygonItem.h

@@ -109,6 +109,8 @@ class UBGraphicsPolygonItem : public QGraphicsPolygonItem, public UBItem
         qreal originalWidth() { return mOriginalWidth;}
         bool isNominalLine() {return mIsNominalLine;}
 
+        void setNominalLine(bool isNominalLine) { mIsNominalLine = isNominalLine; }
+
         QColor colorOnDarkBackground() const
         {
             return mColorOnDarkBackground;

src/domain/UBGraphicsScene.cpp

@@ -893,6 +893,7 @@ void UBGraphicsScene::eraseLineTo(const QPointF &pEndPoint, const qreal &pWidth)
                 UBGraphicsPolygonItem* polygonItem = new UBGraphicsPolygonItem(intersectedPolygons[i][j], intersectedPolygonItem->parentItem());
 
                 intersectedPolygonItem->copyItemParameters(polygonItem);
+                polygonItem->setNominalLine(false);
                 polygonItem->setStroke(intersectedPolygonItem->stroke());
                 polygonItem->setStrokesGroup(intersectedPolygonItem->strokesGroup());
                 intersectedPolygonItem->strokesGroup()->addToGroup(polygonItem);