Set the BuildVolume node to disabled

This prevents accidental modification of the node

cura/BuildVolume.py

@@ -65,6 +65,7 @@ class BuildVolume(SceneNode):
         self._origin_mesh = None  # type: Optional[MeshData]
         self._origin_line_length = 20
         self._origin_line_width = 1.5
+        self._enabled = False
 
         self._grid_mesh = None   # type: Optional[MeshData]
         self._grid_shader = None