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
Merge pull request #497 from justino/composer_from_url

Composer: create from URL