Multiple postprocessors

This changes the actual postprocessor path into a gsetting. On first run the old lookup code is used to set this setting.

On the settings page a new dropdown is added to pick the postprocessing script you want to use. On a pristine install this would just show (built-in) postprocess.sh. If someone created /etc/megapixels/postprocess.sh or ~/.config/megapixels/postprocess.sh then those options will also be shown.

The main reason for this is the addition of /usr/share/megapixels/postprocessor.d/*. Additional postprocessors can be packaged here and they would be user-selectable. These scripts are just shown in the dropdown as their filenames.