Fix four memory leaks found by AddressSanitizer

- StyleEditor: QGridLayout(this) pre-empted the widget's layout slot,
  causing setLayout(main_layout) to silently fail and orphan main_layout.
  Fix: use QGridLayout() without a parent so setLayout() succeeds.

- ExportDialog: ~ExportDialog() was empty, leaving ExportDiagramLine
  heap objects in diagram_lines_ unfreed. Fix: qDeleteAll(diagram_lines_).

- GenericPanel::getItemForDiagram: when called without the bool* created
  arg, it created a parentless QTreeWidgetItem that callers immediately
  discarded. Fix: return nullptr when created==nullptr and item not found
  (all callers already guard with if (item)).

- ElementScene: m_paste_area (created in initPasteArea) was temporarily
  added/removed from the scene during XML loading but never freed in the
  destructor. Fix: delete it if not currently in the scene.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sources/genericpanel.cpp

@@ -321,9 +321,11 @@ QTreeWidgetItem *GenericPanel::getItemForDiagram(Diagram *diagram,
 		if (created) *created = false;
 		return(diagram_qtwi);
 	}
-	
+
+	if (!created) return(nullptr);
+
 	diagram_qtwi = makeItem(QET::Diagram);
-	if (created) *created = true;
+	*created = true;
 	return(diagram_qtwi);
 }