Add function to set key_frame for VideoFrame

[daveisfera]

No description provided.

[WyattBlue]

Issues from least to most important:

1. If key_frame is going to be settable, it should be settable in the base Frame class.
2. I'm not sure why None is a special case.
3. I think this implementation wouldn't work for many video formats
4. The user could just modify the VideoFrame flags themselves, they don't need a special method

[daveisfera]

1. If key_frame is going to be settable, it should be settable in the base Frame class.

I could move this to the base class

2. I'm not sure why None is a special case.

I was just copying the other setters, but could easily treat None like False

3. I think this implementation wouldn't work for many video formats

I've only really worked with the H.264 and HEVC code in ffmpeg, so what specifically wouldn't work with many video formats? Isn't flags and pict_type how the key frame information is communicated?

4. The user could just modify the VideoFrame flags themselves, they don't need a special method

I'm still using 14.4 and it looks like 15 was a pretty big re-structure, but I couldn't access access flags directly when I was testing things. Also, I assumed that have a single property to handle the separate pieces that are implied with key_frame = True was a nice convenience, but I'm glad to directly handle that if it's the better solution.

[WyattBlue]

Superseded by #1965