Defer topic DOM positioning (#505).

View Customization Framework: Initial positioning of the topic DOM is deferred until the "on_update_topic" and "on_update_view_properties" hooks are invoked.

As a consequence the customizer's "topic_dom" code must no longer build up the topic DOM as far as its size is computable (but can rely on its "on_update_topic" and "on_update_view_properties" code for doing so). Thus the view-sync behavoir is called from 1 spot only. View Customizer developement is simplified.

See #505.