Implemented new preference system

To work with preferences, now it's as easy as referencing the preference
names that automatically become part of the `euskalbar.prefs` object as
getters/setters:

  - `euskalbar.prefs.prefName` will retrieve the value stored for the
    preferences.
  - `euskalbar.prefs.prefName = value` will store any type of JavaScript
    value into the preferences backend. Types unknown to the Firefox
    preferences system are encoded as JSON strings.

When using setters, their values are automatically stored in the backend
and the event handlers are triggered accordingly.
Removed subquery support
Removed redundant label
Don't list unsupported dictionaries
Major i18n/l10n cleanup
Cleanup: indentation
Use switch/case statements
Removed not implemented method
Don't break the app if loading a dict fails

When a dictionary fails to load, it'll be removed from the list of
available dictionaries.
Fixed wrong reference