Webclient: extend REST API (#315).

Core API:

The Type interface has a new method:
{{{
boolean hasAssocDef(String childTypeUri)
}}}

Webclient REST API:

A new service call requests a topic's related topics excluding the "property topics":
{{{
GET /webclient/topic/{id}/related_topics
}}}

"Property topics" are child topics that adhere to a parent topic's "Composition" definition.

In preparation of suppressing redundant topics in the detail panel's related topics listing (see #315).
Suppress redundant topics in detail panel (#315).

Redundant topics are suppressed in the detail panel's related topics listing. A topic is regarded redundant if 1) it is already represented in the detail panel's upper area (the "model-driven box structure"), and 2) there is a "Composition Definition" in the underlying model.

Core API:

The Association interface has a new method:
{{{
boolean isPlayer(TopicRoleModel roleModel)
}}}

The REST Client has 2 new methods:

{{{
dm4c.restc.get_related_topics(topic_id, sort)
dm4c.restc.sort_topics(topics)
}}}

See #315.
Webclient: show assoc type in detail panel (#315).

In the detail panel's related topics listing for each topic the respective association type is shown beneath the topic.

Bug fix: tooltips in File Browser's "Folder Content" listing.

DEVELOPER NOTES

Core serialization format (server-side):

- Revert change as of Nov 3 (0c7423ca): a RelatedTopic's association part *is* serialized (again).
So the serialized JSON object *has* an "assoc" property (again).

Webclient API (client-side):

- The Topic constructor adopts the underlying object's "assoc" property as well

- New convenience method:
{{{
dm4c.association_type_name(type_uri)
}}}

BREAKING CHANGE

- Webclient convenience method renamed:
{{{
dm4c.type_label(type_uri) -> dm4c.topic_type_name(type_uri)
}}}

See #315.
Webclient: click on canvas resets selection.

See #523.
Webclient: fix selection reset (#523).

The selection is not reset on mouseup when canvas move was in progress but aborted through mouse leaving the canvas.

See #523.
Webclient: fix model and GUI update
Merge branch 'master' into pax-web