kivy: dim qr code on touch

gui/kivy/uix/ui_screens/receive.kv

@@ -28,9 +28,10 @@ ReceiveScreen:
                 size_hint: None, 1
                 width: min(self.height, bl.width)
                 pos_hint: {'center': (.5, .5)}
+                shaded: False
                 on_touch_down:
-                    if self.collide_point(*args[1].pos):\
-                    app.show_info_bubble(icon=self.ids.qrimage.texture, text='texture')
+                    self.shaded = not self.shaded
+                    self.foreground_color = (0, 0, 0, 0.5) if self.shaded else (0, 0, 0, 0)
 
         SendReceiveBlueBottom:
             id: blue_bottom