[TD]Fix section snapping (fix #15961) (#15450)

* [TD]Add view snapping preferences

* [TD]fix section snapping algo

- snap sections to section normal line.
- snap views to other views in X&Y

* [TD]fix snapping to ProjectionGroups

src/Mod/TechDraw/App/DrawViewSection.cpp

@@ -80,6 +80,7 @@
 #include <Base/Console.h>
 #include <Base/FileInfo.h>
 #include <Base/Parameter.h>
+#include <Base/Tools.h>
 
 #include <Mod/Part/App/PartFeature.h>
 
@@ -1152,21 +1153,15 @@ gp_Ax2 DrawViewSection::getSectionCS() const
     return sectionCS;
 }
 
-gp_Ax2 DrawViewSection::getProjectionCS(const Base::Vector3d pt) const
+
+//! return the center of the shape resulting from the cut operation
+Base::Vector3d DrawViewSection::getCutCentroid() const
 {
-    Base::Vector3d vNormal = SectionNormal.getValue();
-    gp_Dir gNormal(vNormal.x, vNormal.y, vNormal.z);
-    Base::Vector3d vXDir = getXDirection();
-    gp_Dir gXDir(vXDir.x, vXDir.y, vXDir.z);
-    if (DrawUtil::fpCompare(fabs(gNormal.Dot(gXDir)), 1.0)) {
-        // can not build a gp_Ax2 from these values
-        throw Base::RuntimeError(
-            "DVS::getProjectionCS - SectionNormal and XDirection are parallel");
-    }
-    gp_Pnt gOrigin(pt.x, pt.y, pt.z);
-    return {gOrigin, gNormal, gXDir};
+    gp_Pnt inputCenter = ShapeUtils::findCentroid(m_cutPieces, getProjectionCS());
+    return Base::Vector3d(inputCenter.X(), inputCenter.Y(), inputCenter.Z());
 }
 
+
 std::vector<LineSet> DrawViewSection::getDrawableLines(int i)
 {
     // Base::Console().Message("DVS::getDrawableLines(%d) - lineSets: %d\n", i, m_lineSets.size());