Draft: Edit, change startEditing method to accept (obj, node_idx)

This enable to start editing also without an user click event, by just calling the method and specifying the object and the edit point index.
It is used for alternative edit mode (alt_edit_mode) by arc context menu

src/Mod/Draft/draftguitools/gui_edit_draft_objects.py

@@ -290,7 +290,7 @@ class DraftCircleGuiTools(GuiTools):
         if action in ("move arc", "set radius",
                 "set first angle", "set last angle"):
             edit_command.alt_edit_mode = 1
-            edit_command.startEditing(edit_command.event)
+            edit_command.startEditing(obj, node_idx)
 
     def init_preview_object(self, obj):
         return trackers.arcTracker()