Improve XOTcl class and object browser:
- use svg instead of png by default
This leads to better graphic output,
certain stylability via CSS, and less tmp files.
- make classes in class hierarchy clickable to
allow for navigation in the class structures
- contrast class of interest (in focus) against contextual
classes in generated graphs
- make show-object configurable to allow eg. how many
levels of the class hierarchy should be displayed
before and after the class of interest
- don't display per default instance-of relationships to
make the graph simpler