V4l2: move v4l2 camera code from the ffmpeg backend into the library

The v4l2 camera code is unrelated to ffmpeg and can potentially be used
independently from the ffmpeg backend. So let's move it into the
library.

As drive-by do some modernizations of the codebase

Pick-to: 6.12
Change-Id: I918ef9004cebd8a0446e53f634a551bb778d7c0b
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>