Add a - None - option for caption selection in field formatter

Fixed, please review!

@Grevil could you please test it and provide a screenshot for @thomas.frobieter to do the final review?
Did you test manually, that it works? Should we add a simple test case perhaps?

Already tested! Works great :)

Okay, let's trust these words ;) Code is simple here.

Automatically closed - issue fixed for 2 weeks with no activity.