Live graph movie functionality upgrade.

This upgrades the old "live graph movie" functionality, by which the gcylc graph view writes out a dot-language file out every time the suite state changes, for post-processing by a script in ```dev/bin/```.

Changes:
 * write dot files to the suite share directory instead of the suite definition directory.
 * upgrade the post-proc script to make it more user-friendly, work for any number of files (the frame numbers have to be padded appropriately), and generate a .webm movie automatically if the right tools are installed (webm can be played natively by Firefox).

This should probably be an obscure gcylc menu option not a suite definition item (the frames were originally written out by the suite daemon).

I've found this useful at times, to show suite evolution in presentations.

@matthewrmshin - please review.  Low priority, doesn't really affect users.  I'll email an example movie.

Done.