rtf filter: port the rtf import filter to Brad Hard's new rtf-qt library

Use the new rtf-qt library to parse rtf. This library parses (for now)
into a plain QTextDocument and I use QTextDocumentWriter's ODF capabilities
to export to ODT.

The filter is more limited than the previous one, perhaps, but has several
important advantages:

* it generates ODT, so it is actually useful
* it is based on nice, clean code that has been touched in the last ten
years

In the future, we need to remove our copy, when rtf-qt gets packaged.
CCMAIL: Brad Hards