Big Commit (TM):

- made the XML format cool. Now the XML file contains lots of
information about the library, e.g. data types, categories, command
parameters etc. The XML file also contains english user-visible strings,
which can eventually be extracted for translation.
- extended the *Description classes such that they hold all needed
information. Also, XML writing is supported now.
- DatatypeDescription and CategoryDescription are gone now. Datatyps and
categories describe themselves.
- datatypes and categories are now created automatically, as well as
command parameters and results, reducing code size dramatically.
- extended the command API a little.
- all commands are ported to the new API.
- other small fixes and improvements.