Rewrite history detail view

This change is a total rewrite of the current history view.

Instead of drawing it all ourselves, setup some HTML and show that in a
browser. This addresses most of the details related topics in
macgitver/MacGitver#3:

- Details view becomes scrollable
- Details view becomes selectable
- Lineheight for fixed-font header lines is fixed.
Accidently deleted some methods before committing
Don't include QPainter