EXPORT store appearance status in Gexf

There is a GEXF spec version to store caption information.
Gephi Lite is ready to use that spec. To do it we need to:
- implement the new gexf version in graphology
- use it in gephi lite

Start by locating GEXF export handling in graphology and the corresponding import or display path in Gephi Lite. Check the referenced GEXF specification for caption and appearance status support; the work is done when graphology exports the new version and Gephi Lite uses it.

Written by the indexing model from the issue text.