Plugins weren't being held by their packages, just by their names.
I fixed this by mapping package names to plugin names and nice
names. The PreferenceManager has been updated to search for the
appropriate plugin name. Plugin real names can be duplicated in
different packages.

- Fix missing config file behaviour
- Remove misc debug prints
- Add folder path preference to document plugin
- Change PluginSelectDialog to show nice plugin names
- Add support for real *or* nice names in most PSD functions