Topicmap renderer refactoring, pt.1 (#505).

BREAKING CHANGES

In preparation of the imminent refactoring of the topicmap renderer framework some client-side classes are renamed:

    Webclient:
        DefaultTopicmapRenderer  -> CanvasRenderer
        DefaultTopicmapViewmodel -> CanvasView
            CanvasTopic -> TopicView
            CanvasAssoc -> AssociationView

    Topicmaps:
        Topicmap            -> TopicmapViewmodel
        TopicmapTopic       -> TopicViewmodel
        TopicmapAssociation -> AssociationViewmodel

    Geomaps:
        Geomap      -> GeomapViewmodel
        GeomapTopic -> GeoTopicViewmodel

=> Plugins which provide a topicmap renderer implementation must be adapted.

Furthermore one object is renamed:

    dm4c.canvas -> dm4c.topicmap_renderer

=> Plugins might require to be updated.

See #505.