Implement rendering switch (normal <-> monospaced font)

[acautin]

The switch can be similar to the graph view toggle.

From #573 (comment)

We are mainly comparing code here. This is the main purpose of the new diff function. That said, we would like to have the possibility (maybe in general) to switch between tow different renderings:

• html view (normal dderl rendering with compressed white space and hidden line breaks)
• fixed font view with all white space rendered (tabbing on 4 char lines)
  Please create a new issue for this if it cannot be done in this one.