ConnectMode: right click to cancel connection

SketchWidget.py

@@ -630,4 +630,7 @@ class ConnectMode(Mode):
                 sw.invalidate()
 
     def do_right_click(self, sw, x, y):
-        sw.window.set_mode('')
+        if self.first_ptref is not None:
+            self.first_ptref = None
+        else:
+            sw.window.set_mode('')