Pitivi should use Gtk Composite Templates
Gtk Composite Templates has been there for a long time (for Vala for example). Python bindings did not have support for it. However, since some years ago it is supported and we should use it.
Advantages of using it: we will reduce LOC (lines of code) and move code to Glade ui files. We can do quick changes there related to design without modifying code. Disadvantages: We will end up using Glade a lot more... and the more branches on a same glade file, resolving Glade conflicts may be a pain in the neck.