NavigationBar and some fixes

Hi,

I know your project is called `clean-status-bar` but I have to admit, it would be awsome to include the navigation bar as well.
I've got an HTC One (M7) and this device doesn't come with a Navigation bar.

So maybe your app could be renamed to something more generic like : `clean-system-bars`
I don't want to force you to do it, but I've already do it on my fork, so if you appreciate this work, feel free to accept the pull request ;)

I've also added some fixes:
- use strings resources for preference keys
- disabled status for your ColourPreference
- better managment of preference dependencies