Fix bug 1445435: Extend access key candidate generators (#899)
Fix bug 1445435: Extend access key candidate generators (#899)

Generate accesskey candidates from textarea elements belonging to the
"label" attribute. If such attribute doesn't exist, generate candidates
from value elements.

Also:
- Exclude space as a possible accesskey candidate
- Add more accurate current accesskey detection
- Make access keys case sensitive

Case sensitive access keys
mozilla/pontoon,mathjazz/pontoon,jotes/pontoon,mozilla/pontoon,mozilla/pontoon,mathjazz/pontoon,mathjazz/pontoon,jotes/pontoon,mathjazz/pontoon,jotes/pontoon,mozilla/pontoon,jotes/pontoon,mozilla/pontoon,mathjazz/pontoon