Clipping changes.

* Save the flipped state of the aspect ratio preset along with the preset
  itself.  (plugins/darkroom/clipping/aspect_flipped)
* Change the saved custom_aspect to be a string with the actual custom
  aspect.  This is so the user will see what he/she entered instead of a
  computed value.
* When the user enters an aspect ratio of his/her own, actually save it
  too.  (plugins/darkroom/clipping/custom_aspect)
* Handle the saved presets better when starting up.

This fixes some of the issues I have reported.  Unfortunately, I can't reach redmine to get the numbers right now.