Hardcode the order of the sections in the Preferences dialog
Currently the order of the sections in the Preferences dialog is random, but it should be hardcoded.
See PreferencesDialog.section_names
in pitivi/dialogs/prefs.py for the set of current sections, and how that is iterated to create the UI. Note the solution must handle well new sections created dynamically by plugins which can be loaded at any time, for example.