OpenAPI Support: Enable right-click LEMMA model generation

[frademacher]

Currently, the OpenAPI plugin requires the user to open a dialog and select an OpenAPI specification file next to having to input some additional stuff like model names. To facilitate the generation of LEMMA models from OpenAPI specification files, it would be nice if the user could right-click on an OpenAPI specification file in the Eclipse Package or Project Explorer, click on an entry like "Extract LEMMA Models from OpenAPI Specification", and get the OpenAPI generation dialog opened with the specification file text field pre-filled with the path to the right-clicked file.

A good starting point for the integration of right-click support in Eclipse' Package and Project Explorers is

lemma/de.fhdo.lemma.data.avro/plugin.xml

Line 32 in 4c1de5a

<extension