Use gtksink and autovideosrc instead of xvimagesink and v4l2src

Tobias Mueller requested to merge gh-eb66e79e/2/muelli/gtksink into master

Created by: muelli

The gtksink helps us to remove code dealing with the X window IDs and more. It should also work much more reliably in virtual machines or with GL sinks.

The autovideosrc should work nicely with sandboxed environments like Flatpak.

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