Improved RTF output

See #1145

This is basically a refactoring of the existing code. I only touched **dita2rtfImpl.xsl;** all other files in the `xslrtf` sub-folder still remain unchanged.

## Changes
- Removed unused settings (fonts, colors etc.) and streamlined the remaining ones.
- Moved most of RTF specific code to an external include file.
- Removed most inline styling.
- Started wrapping paragraphs in {\pard ... \par} for better maintainability.
- Added some default settings.

## To do
- [ ] Improve list styling, especially for steps.
- [ ] Refactor table styling.
- [ ] Improve page layout. 
- [ ] Add RTF documentation.