Fix BioPortal Import plugin

Main changes:
- Update the "search" and the "view concept details" functionalities to work with the new BioPortal API
- Improve pagination of search results, using "previous" and "next" buttons instead of "show all results"
- Adapt the ontology visualization functionality to work with the new visualization service used by BioPortal
- The class tree is immediately refreshed when importing a class

Note: The plugin is using the "ontologies_api_java_client" API (https://github.com/palexander/ontologies_api_java_client) to access BioPortal but that API has not been published yet in Maven Central. This is an open issue (https://github.com/palexander/ontologies_api_java_client/issues/1)