Restructure and clean up the video/video_reader C++ implementations

[datumbox]

🐛 Bug

The C++ implementations of video and video_reader need to be refactored and cleaned up similar to #2998, #3051, #3094, #3097, #3135, #3163:

• Rename C++ files, methods & variables according to recommended naming conventions.
• Move C++ code away from header files and into cpp files.
• Ensure each cpp file has a separate header file with "public" functions.
• Reduce unnecessary header inclusions.
• Add all internal functions in anonymous namespaces.
• Sync variable names between the cpp files and their header files.

[fmassa]

For the decoder folder, I recommend leaving it unchanged and not performing any changes to its code.

My thinking was that we should instead make things a bit more organized in the sense that it now seems to be 3 different projects, but instead decoder is a generic abstraction around ffmpeg and the other two leverages decoder to expose ffmpeg functionality to Python.

My thinking was to re-structure those three libraries to instead have a more natural flow.

[datumbox]

@fmassa Sounds good. I've updated the issue description to reflect that.