Refactor the uploader code into a nicer module.

* xmlrpc usage encapsulated
* utility functions grouped
* UI and Operators separated
* Encapsulate exceptions

As with previous commit, this version now utilizes the new way
of storing user credentials, making it a lot easier for users
to use the uploader in subsequent usages.

Note that refreshing the session list isn't done on login anymore
so remember to press that button :)