Move element specific components to sub folder and just generally 
improve the file/component structure.

#552

#610

#618

#608

- Prevent input of 'Dead' keys

(which is the height of a single item)

#525

- and AnswerSchemeValue with VariableValue

- Remove unused imports

#599

- Now every element can overwrite 'getAnswerScheme' of 'UIElement'

- Method is already defined in superclass

#583

#596