Remove unused code.

This is unused since change
2013-05-28T17:04:12Z!h.goebel@crazy-compilers.com, but I missed removing.
Fix: oowriter exporter is listed twice in export dialog.

This is caused by XsltExportEngineFactory, which includes all
files matching "mm2([\w]+)\.xsl" into the export list. While this is
conveniant, exporting only the content.xml of a ODT is quite useless.

Renaming `mm2oowriter*.xslt to `export2oowriter*.xsl` solves this.
New: FileUtils.slurpResource(): read resource into a string.

Also introduce a prvate method FileUtilsl.slurp() for reading
data from any Reader into a string.
Read javascript for folding HTML from a new resource `/html/folding.js`.
Read CSS stylesheet for HTML from a new resource `/html/folding.css`.
Cleanup: Unite common CSS property-definitions for selectors.
Merge branch 'work-on-html-export' into master.