Read out view properties at client-side (#505).

View Customization Framework:

The canvas view facade passed to a view customizer constructor has another property:
{{{
get_topic -- a function with a "topic_id" argument
}}}
The view customizer author can call this function to access a particular TopicView object e.g. to read out its view properties.

For an example see the Box Renderer:
https://github.com/jri/dm4-box-renderer

See #505.