Fix missing DWG imports in ArchSectionPlane.getSVG

When a DWG file is imported into FreeCAD it gets imported as a Part
object with a compound shape. One would imagine, that such an object
should be rendered like a symbol inside a TechDraw View. But when the
imported shape is inside a SectionPlane that also contains solids, the
shape is not included in the SVG.

The problem is, that getCutShapes only uses solid shapes, when a volume
was cut before. But the imported DWG is a flat 2D object and has no solids
inside.

To fix it, the getSVG method will check if an object looks like a draft
object, meaning:
 - It has a shape
 - It has no solids
 - It has no volume

When it finds such a shape, it will treat it like a draft object. So
it is displayed like a symbol in the resulting SVG.

https://forum.freecadweb.org/viewtopic.php?f=3&t=34079

src/Mod/Arch/ArchSectionPlane.py

@@ -95,6 +95,18 @@ def makeSectionView(section,name="View"):
     view.Label = translate("Arch","View of")+" "+section.Name
     return view
 
+def looksLikeDraft(o):
+    # If there is no shape at all ignore it
+    if not hasattr(o, 'Shape') or o.Shape.isNull():
+        return False
+    
+    # If there are solids in the object, it will be handled later
+    # by getCutShapes
+    if len(o.Shape.Solids) > 0:
+        return False
+    
+    # If we have a shape, but no volume, it looks like a flat 2D object
+    return o.Shape.Volume == 0
 
 def getCutShapes(objs,section,showHidden):
 
@@ -178,6 +190,8 @@ def getSVG(section, renderMode="Wireframe", allOn=False, showHidden=False, scale
             drafts.append(o)
         elif o.isDerivedFrom("Part::Part2DObject"):
             drafts.append(o)
+        elif looksLikeDraft(o):
+            drafts.append(o)
         else:
             nonspaces.append(o)
         if Draft.getType(o) == "Window":