Composer: create from URL

New function in the composer toolbar to create a graph from a URL.
This is based entirely off of @obfuscurity Stupid Graphite Tips
https://speakerdeck.com/obfuscurity/stupid-graphite-tips?slide=12

Icon is Creative Commons:
https://www.iconfinder.com/icons/49529/link_url_icon

This is something I've seen requested a few time on the IRC channel

This PR is based on branch 0.9.x