Screenshot support

[tomstitt]

Press S to screenshot in live/index.html

Depends on GLVis/glvis#195

TODO:

• pick filename
• verify pyglvis support

[tzanio]

The name that the desktop version uses is GLVis_s01.png and then GLVis_s02.png for consecutive screenshots in the same Window.

I don't think we should do the same though, here glvis.png and relying on the browser to save to a different file, e.g. glvis (1).png on a Mac seems better.