Implement desktop notifications, implement caching in CLAPI

- Switched to the unminified version of normalize.css
- Implemented desktop notifications, which can be enabled via the options.
- Fix styling issues with Chrome (searchbar position).
- Implemented a brand new caching system in CustomLangsAPI: loaded languages are stored in the localStorage for 6 hours and then reloaded. Can be cleared with a new public method. Documentation updated.
- Removed an useless linebreak in the preferences layout.
- New PreferencesAPI parameter in the settings structure: 'display', which allows to completely ignore an option. Useful when you need to completely disable an option if the user does not meet a specific criteria.
- CustomLangsAPI can now be initialized twice: the languages will not be reloaded and the callback will be called immediately.
- CustomLangsAPI now registers a global Ajax listener when the user is in the options. It is used to clear the cache when the user changes its language.