XHTML output for InsetInclude.

XHTML output for InsetInclude.

Here's the deal:
* With verbatim, we include it verbatim. This would allow the inclusion of
  other HTML files.
* With listings, we include it verbatim, wrapped in <pre>.
* With Input and Include, we check if it's a LyX file. If not, we don't do
  anything, since we don't know how to include (say) a TeX file in the HTML
  output. (Wanna call tex4ht, anyone?) If it is a LyX file, we let it write
  itself as HTML, and include it.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30191 a592a061-630c-0410-9148-cb99ea01b6c8