Port UI Files to Blueprint

This merge request ports all XML-based UI files to Blueprint. Blueprint makes defining user interfaces much more convenient and flexible and it is being adopted by more and more GNOME-related projects.

In addition to porting some of the XML-based files, I also ported some of the UI defined within the Vala files to use Blueprint. However, not everything has been converted since the current layout of the code makes this not particularily straight-forward in a few places.