Fix dragging parts selected in the symbol editor's Parts list

Selecting several parts by clicking them in the Parts list, then dragging
one on the drawing, moved only the part under the cursor; the others
stayed where they were. No selection handles were shown either.

updateSelectionFromPartsList() blocks the scene's signals while it copies
the list selection to the drawing, so selectionChanged never reached
ElementScene::managePrimitivesGroups(), which sets up the decorator that
moves a multi-part selection. Call it once the signals are unblocked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/editor/ui/qetelementeditor.cpp

@@ -765,6 +765,9 @@ void QETElementEditor::updateSelectionFromPartsList()
 	}
 	m_parts_list -> blockSignals(false);
 	m_elmt_scene -> blockSignals(false);
+		//selectionChanged was blocked above, so the selection decorator must be
+		//updated by hand, otherwise dragging moves only the part under the cursor
+	m_elmt_scene -> managePrimitivesGroups();
 	updateInformations();
 	updateAction();
 }