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.

Snapshot from a Linux and Qt4 based build:
![History_Details](https://f.cloud.github.com/assets/1700771/238985/1a65e14e-8852-11e2-8219-9eb7347e7912.png)