Drop QVideoWidget.

QVideoWidget is based on GStreamer and horribly buggy, so replace it with
our own homegrown code. This reuses and adapts a lot of code from Nageru
and Movit, but especially the seek code is pretty much written from scratch.

The biggest advantage is that we no longer get random hangs all the time
(which the F5 key would attempt to work around, by restarting the player).
The disadvantages are that we take on a bunch of new code, a direct dependency
on FFmpeg and OpenGL (4.6 for the time being, but can probably be reduced
to much lower if we want to target macOS), and lose sound support.
We get VA-API/VDPAU acceleration, although I'm not sure if the old code
did that for us or not.

The other motivating change is that this will allow us to implement zoom
more easily down the road.

The frame-based seeking is now actually frame-based, instead of trying to
emulate it by going +/- 20 ms.