Add protocols to external link picker

See the CMS 6 screenshot.

Add a list with protocols to pick from:
http://
https://
mailto:

Set "http://" as the default selection. The actual value stored in the link should include the protocol (ie, it should be stored as "http://www.cnn.com" not "www.cnn.com").
When an existing link is edited, parse the protocol, select the right value in the dropdown and show only the url in the inputbox (ie "www.cnn.com").