Added position and slide tracking for reveal.js

The reveal.js editing feature is promising.
Unfortunatly the preview jumped to the first slide on every edit. And for large presentations it's hard to find the slide in the markdown.

I added both features with this pull request.
The code is a little rough, as everything is done in the mainwindow. But the functionality is there and works.
I thought about that already. But this would also be part of any html export.