new feature: GeoGebra tag: Combining GeoGebra and STACK

adding a new feature to create tasks with GeoGebra:
GeoGebra tag is based on JSX Graph implementation and adds the possibility to create GeoGebra tasks with STACK feedback.
The new tag features special js commands: stack_geogebra.bind_point(), stack_geogebra.bind_value(), stack_geogebra.bind_point_to_remember_JSON(), stack_geogebra.bind_value_to_remember_JSON()
The new tag features sub-tags: set, watch and remember

The new tag features global defaults for GeoGebra: path to GeoGebra

How to use the new tag and its sub-tags? See documentation GeoGebra.md

In this commit also code is added for further development: preparing special GeoGebra input type, which is not used yet by the geogebra tag