Webclient: fix deleting types (#246, #608).

Deleted types disappear from type menus (#246).

2 new client-side events (#608):
{{{
post_delete_topic_type
post_delete_association_type
}}}
Fired once a type has been deleted. The type URI is passed to the listener.

Webclient API has 2 new methods:
{{{
dm4c.do_delete_topic_type(type_uri)
dm4c.do_delete_association_type(type_uri)
}}}

REST client has 2 new methods:
{{{
dm4c.restc.delete_topic_type(type_uri)
dm4c.restc.delete_association_type(type_uri)
}}}

Close #246.
Close #608.