Adopt Clapper for Video + GIFV playback

[diamondburned]

Tuba did this: GeopJr/Tuba#931

Adopting Clapper would let us stream videos and GIFVs instead of downloading whole files and playing them back afterwards. It also helps us cut off a lot of bloat code to download stuff properly.

We might even be able to use this for future audio playback support too.

[diamondburned]

Created branch https://github.com/diamondburned/dissent/tree/embed-refactor in case we decide to do this. Also will need a gotk4-clapper generated.