Allow mapping export

[paulzierep]

It would be convenient to allow the export of the mapping (connection between the drawIO rectangle and the column), so that it can be used automatically for new data. It would also be useful if this mapping could be imported to drawIO so that it can be used again.

[yuechenbam]

We have to choose a language for the export of mapping. Maybe RML, so the exported data can be used more standardly. Otherwise we could use JSON format, but in this case, the file can be only used by this Ontopanel. This idea is good and is actually one of the development plans in the future.