Extend server-side Topicmaps API (#544).

CHANGES

- `TopicmapViewmodel` has 2 new methods:
    - `getTopics()` returns an `Iterable<TopicViewmodel>`
    - `getAssociations()` returns an `Iterable<AssociationViewmodel>`

- `TopicmapsService` has a new method (not RESTful):
    - `boolean isTopicInTopicmap(long topicmapId, long topicId)`

- `TopicmapsService` has a new convenience method (not RESTful):
    - `void addTopicToTopicmap(long topicmapId, long topicId, int x, int y, boolean visibility)`

- mew package/classes:
    - `de.deepamehta.plugins.topicmaps.model.TopicViewmodel`
    - `de.deepamehta.plugins.topicmaps.model.AssociationViewmodel`

BREAKING CHANGES

- `TopicmapViewmodel` moved package:
    `de.deepamehta.plugins.topicmaps.TopicmapViewmodel`
    -->
    `de.deepamehta.plugins.topicmaps.model.TopicmapViewmodel`

See #555.