nick/settings-manager needs testing

[Nick Schermer]

Last evening I've pushed the category-based settings dialog. Maybe
some folks can give it a shot.

Big changes:
- Category-based.
- Uses Garcon for items
- Pluggable dialogs by default (you can't disable it).

Todo:
- Need to restore the --dialog option.
- Maybe I add middle click to open-without-pluggable.

Everything else should be working at it was.

To test the dialog (nick/settings-manager branch), you also need
garcon from master (2 new .directories required) and exo for some
fixes and new api in exo-icon-view.

Nick