Good job Joerg. Maybe we put you in charge of the Linux builds from now on.

I'm not sure the video streaming (behind HUD) in QGC ever worked, and I'm pretty sure it won't work now. I've never really played with it myself. There's an option Peter added at one point to build with VLC media player support embedded, but I've never gotten it to work myself and it is not included in the standard builds (the VLC library files are double the size of the complete QGC distro... I'm sure there's a more efficient option to play media).
Qt5 multimedia capabilities are much improved over Qt4 and I would imagine it shouldn't be difficult to add the video underlay functionality back into QGC (since the "stubs" are already there). Or just a new video player widget. I'd welcome any contributions! For me personally it's not a requirement... The QGC widgets can always be displayed on their own (outside of main QGC window), eg. surrounding a video player window or whatever. The embedded option really makes sense if you want to overlay other data on it (like HUD), or interact with the camera via Mavlink, or something like that.
As for sending images over Mavlink (or, worse, video), I just don't see how that would work very well... except maybe in certain limited circumstances or with very high bandwidth available (like Ethernet or good WiFi). The delay would be long, and the error checking is minimal which could easily distort images. The rest of the telemetry stream would suffer badly -- there's no way to prioritize messages/packets in Mavlink.
I think something with high bandwidth requirements like this pretty much needs it's own data stream (own radio or shared over a proper network). And even over a relatively fast connection like USB, the Mavlink protocol simply has too much overhead to make it very practical for this (I've pushed the limits with an AQ M4, and they're fairly low). Heck, even, for example, browsing a collection of images on a thumbdrive over USB2 is pretty slow and annoying... only USB3 makes it really practical. Imagine doing that over a 115K baud serial radio stream?
Cheers,
-Max