SVGHMI: add support for "reference" and "frame" rectangles to spread-out ovelapping elements.

When HMI become complicated, designer needs to spread-out ovelapping elements
in order to unclutter drawing and facilitate maintenance.

SVGHMI: remove intermediate "updates" Map and apply_updates()

It was used initially to decouple DOM updates from reception of data through
websocket, but now since all widget SHOULD use animate() to modify DOM, and
dispatch() only change internal state, apply_update is unnecessary.