Need all folders, new message in block links

Currently, the Privatemsg links block only lists the Inbox. Obviously, the Sent folder should appear here, as well as user-created folders when that feature is implemented. Here is a patch to add the Sent folder, as well as a link to create a new message. There's probably a prettier way of doing this; let me know if so.

I'm partial to the change I made from the current Navigation menu item order, with the "Create private message" item first (and mimicking other "Create [content-type]" menu items) and the folder list following. I've also separated the folder list from the new message link to differentiate between the two types of links in the block. If this isn't appropriate let me know and I'll change it.