Feature: WAI-Aria
- Adds wai aria tags to those elements where it is necessary to gain a semantic structure
- Adds tab indices to the quiz manager, the new answeroption elements and the question type selection
- Adds an answeroption via clicking on the new answeroption element (usable without mouse)
- Replaces all 'var' variable declarations with the ES6 standard 'let' or 'const' (not exactly a topic of the WAI-Aria since they get backported by babel-runtimes but for the this is an improvement in semantic code structure)
Fixes #506 (closed)