Topicmaps: refactor class names (#505).

BREAKING CHANGES:

Package de.deepamehta.plugins.topicmaps.model is dropped.
2 classes are located in the "topicmaps" packgae directly:

    - de.deepamehta.plugins.topicmaps.TopicmapViewmodel
    - de.deepamehta.plugins.topicmaps.ClusterCoords

TopicViewmodel and AssociationViewmodel are now private inner classes of TopicmapViewmodel.

See #505.