Add option to copy (colored) document body only

Add an option that allows to export colored source code
- using CSS
- but only export the `<pre>...</pre>` element
- without a complete document

This makes sense in an environment where the CSS is already present as
an external style sheet (e.g. in the Lilypond blog ...)

This was present in my own earlier implementation of the source export dialog, and while I'm not sure if there are *that* many users having that situation I think it *is* a good addition.