Improve accessibility in Drag Text

This has been started on in so make sure that the code from that issue is reviewed as well.

A couple of notes to take into account when continuing:

• Try to be surgical – have a high threshold for refactoring. We want to break as little as possible, especially when it comes CSS as people(and content types) may already have overridden styles.

• We should stick to the one event system/interface that H5P is offering – this is to avoid any confusion, duplication of code, bugs etc.