Feature: Control animation speed

[spanag]

It makes sense to view an animation at different speeds, for example to "zoom in" in time during interesting events. Currently, animated properties are declared as dicts of real time to associated values. This means that changing animation speed means rebuilding the dicts, re-transmitting and resetting most of the display, which can become cumbersome (either in time or extra coding) and precludes offline publishing.

Instead, it would be simpler to add a speed control alongside the time slider and scale animation's increments by this factor; fixed stepping would also be nice to have (see #458).