Span property group headers across all cells to avoid text truncation

Render the background of property value editor widgets to avoid cell contents showing through the editor

src/Gui/propertyeditor/PropertyEditor.cpp

@@ -203,6 +203,10 @@ void PropertyEditor::setEditorMode(const QModelIndex & parent, int start, int en
         if (propItem && propItem->testStatus(App::Property::Hidden)) {
             setRowHidden (i, parent, true);
         }
+        if (propItem && propItem->isSeparator()) {
+            // Set group header rows to span all columns
+            setFirstColumnSpanned(i, parent, true);
+        }
     }
 }