Waveform Simulation - perform waveform SVG creation in background

[tomcl]

The waveform SVGs take some time to generate. Currently when waveforms are selected the generation happens immediately which leads to an unresponsive selector when a large number of waveforms are selected at once.

Also, when waveforms are scaled or panned , more generation is sometimes needed, which again takes time and makes the simulation view unresponsive. (Though this is probably less annoying than the selector).

Both problems can be solved by:

• Creating wavefom SVGs as a sequence of separate "create next batch of SVGs" messages each no longer than say 50ms
• Scheduling any outstanding SVG creation after small gap of say 10ms. This will allow other events to be processed and screen to update
• For the case where not all SVGs have been created and the waveforms are visible display those that are created with white screen for those that have not yet been created. (Mayeb it would be less distracting to display the old SVG, if this exists, TBC).

This issue is linked to #298

[tomcl]

see also #332 - it turns out that SVG creation is at the moment very inefficient an can quite easily be speeded up.