Add a way to get the filmstrip widget in the proxy.

This is needed to connect a DnD signal to it.
Remove an image from the map when dropped into the filmstrip.
Merge branch 'remove-from-map'

* remove-from-map:
  Remove an image from the map when dropped into the filmstrip.
  Add a way to get the filmstrip widget in the proxy.
Overexposed is back in the pipe

but now as a gui-less IOP that is always on (like gamma). I also added a
shortcut ('o') to toggle overexposure indication. Now that we no longer
change the history stack that is safe.
opencl: make overexposed work again
overexposed: take care of legacy params
Merge remote-tracking branch 'upstream/master' into 500px
The auth step should work... if 500px were not returning a http error 500