Reposition topic DOM on view prop update (#505).

View Customization Framework:
After invoking the "on_update_view_properties" hook the framework adjusts the position of the topic DOM. This allows view customizers to perform size affecting topic DOM manipulations within "on_update_view_properties".

E.g. when an expansion handle (whose state is stored as a view property) is clicked the size of the topic DOM changes. The customizer must not care about repositioning the topic DOM.

See #505.