Webclient: HTML based topic rendering pt.5 (#505).

- Hiding and deleting topics works.
- Creating associations works (both, via content menu and via mini icon).

View Customizer Framework:

1. Another hook for HTML-based topic rendering:
{{{
topic_dom_draggable_handle(topic_dom, handles)
}}}
By using this hook a View Customizer can control which part of the topic DOM can initiate dragging, e.g. the Box Renderer excludes the topic's mini icon from being draggable as it serves for creating associations. By default the entire topic DOM can initiate dragging.

2. The canvas view facade (passed to a view customizer constructor) has another property:
{{{
pos -- a function that takes an "event" argument
}}}
View Customizers can call this function to convert a mouse event into a canvas coordinate. This works also if the event occurred in any element of the HTML topic overlay.

BREAKING CHANGES

Changed signature of one Webclient controller method:
{{{
dm4c.do_create_association(type_uri, topic_id_1, topic_id_2)
}}}
Now it takes 2 topic IDs (instead of a single topic) and can connect arbitrary topics (instead of only the selected one).

See #505.