Also switched from GtkProgressBar to GtkLevelBar, which seemed more appropriate. The number of levels was copied from the GNOME audio settings page.
Also switched from GtkAspectFrame to GtkPicture, as it is likely that GStreamer will use a GdkPaintable or some such in the future, based on the blog post we've found.
More work for the devices can be done later, but needs some internal rework for list models first.

Changed some options, tried the Test Audio button, which seemed to work. Also tried Test Video option, but it fails as expected due to using GTK3 GStreamer widgets.