WIP librem5: Use playbin3 whenever possible
This has the potential to confuse applications (see fd294179) so apps should opt in themselves when possible.
I'm mostly putting this here to document that this is needed to make hantro fast. With this both
gtk4-demo have good video performance, see todd/Stream#5
Marking as WIP since chaneging the playbin globally confuses lots of things atm so apps should set it whenever possible.