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.