Allow users to save images as SVG

[ValeriaVG]

I think it'll be nice to have pixel-art SVG icons and therefore, the editor should be able to:

• group pixels into shapes
• write shapes as SVG paths (e.g. <path d="M150 0 L75 200 L225 200 Z" />)
• add proper colours to paths
• wrap with SVG boilerplate
• save as a file