I18n preparations and German translation

I've moved plenty of hard-coded strings (mainly in layouts) to strings.xml and did a 100% translation to German for those strings.

The app is now translated to German for:
* Main screen
* Editor (with some exceptions: mainly used enumerations with strings, which is a "sub-optimal" thing anyway)
* Preferences

Further: 
* I renamed `newHUD` to just `HUD` (it was referenced as `HUD` in a layout anyway).
* I've fixed some typos in the code

N.B.: Maybe we should introduce a "language selector" in the preferences. I don't know if all the German users will be happy if the app is in German now because pilots are used to English :smile: