Factor out code to generate pango span tags
Improve highlighting of a corner case

In substitution a case difference is now shown with a more subtle markup
so that the idea is conveyed that the change is not a "full" replacement.
Improve highlighting of a corner case

In replacement we don't show the deleted part (it is in the current
source text). However, it is confusing if a very short string (say a
full stop) replaces a much longer string, since it might look as if
the full stop was merely added.