Make question_edit_form more suitable for inheritance

I'm glad to say, this is the last sub-task there for now.

Well, the matter turns out more complicated. A goal is to create a default implementation of definition_inner function which can suit all questions with repeated answers blanks (at least to override with a parent call); it'll get particulars of questions answer's blank as an array from a special function, overrided in a question.

Also there will be relevant set_data and validation things.

Benefits:

• huge code duplication reducing (6 qtypes can benefit from it)
• less qtype-specific strings hardcoded in relevant classes, so qtypes forms are more open to inheritance
• 5 less repeat_elements calls, so less code to modify for MDL-17064