Fix System workspace (#592).

When no user is logged in while the System workspace is selected for the first time the Webclient throws no error.

Managing the workspace default topicmap ("untitled") has changed:
    - Each time a workspace is created (via Workspaces API) a default topicmap is created as well and assigned to the new workspace automatically. This works also outside a request scope (while system startup).
    - There is no "global" default topicmap anymore. The URI "dm4.topicmaps.default_topicmap" has no special meaning anymore.

IMPORTANT

When you create a workspace via REST API you must suppress automatic workspace assignment by adding a query parameter:
{{{
POST /workspace/...?no_workspace_assignment=true
}}}
Future versions of the REST API might not require the query parameter.

CHANGE in Topicmaps API

2 public constants in `TopicmapsService`:
{{{
String TopicmapsService.DEFAULT_TOPICMAP_NAME     (value is "untitled")
String TopicmapsService.DEFAULT_TOPICMAP_RENDERER (value is "dm4.webclient.default_topicmap_renderer")
}}}

BREAKING CHANGE

1 method dropped from Topicmaps API:
{{{
Topic createTopicmap(String name, String uri, String topicmapRendererUri)
}}}
Creating a topicmap with a particular URI is not supported anymore.
Use the 2-arg `createTopicmap()`.

See #592.