Do not convert SVGs to JPEGs when editing with external tool

This patch adds support for opening SVGs in the user's default SVG
viewer/editor. If the image cannot be exported to SVG directly, it is
exported as PNG instead. In no case will an SVG or an unrecognized
format be lossy-converted to JPEG.

Change-Id: I46dc8548c3cfb4d24a4a0078d4e3d7eee8367b1c