Allow for the icon to fall back to PNG if for some reason the SVG icon
fails (as it does on some older Linux systems).

This should fix cylc/cylc/#440.

Revert "Use Ben's new svg icon in gcylc."

This reverts commit a499141d0e7fe47b3de351e0a3331c1d7ee579fa.

Add back the SVG code with a PNG fallback.