Log FFmpeg version and license during startup

Logging FFmpeg version and license information during application startup makes it easier for application developers and end users to see that the FFmpeg media backend is used and which version of FFmpeg is being used. 

Example output:

Using Qt multimedia with FFmpeg version n7.1-22-g11e8319b8e LGPL version 2.1 or later
Available HW decoding frameworks:
     d3d11va
     dxva2
     d3d12va
Available HW encoding frameworks:
     d3d11va
     dxva2
     d3d12va