Topicmaps: make TopicmapViewmodel portable (#612).

The class TopicmapViewmodel can be used by a 3rd party webclient.
To do so it must pass its RESTClient instance to the TopicmapViewmodel constructor:
{{{
var restc = new RESTClient()
var topicmap = new TopicmapViewmodel(1234, {is_writable: true, customizers: []}, restc)
}}}

In your plugin's main page don't forget to load 3 scripts from the standard distro:
{{{
<script src="/de.deepamehta.webclient/script/util/rest_client.js"></script>
<script src="/de.deepamehta.webclient/script/model/topic.js"></script>
<script src="/de.deepamehta.topicmaps/script/topicmap_viewmodel.js"></script>
}}}

That is the DM4 Webclient and DM4 Topicmaps modules must be part of your DM installation.
Alternatively you could include these 3 scripts in your plugin.

See #612.