Add ability to copy metadata from input sources to the output one.

Metadata aware objects:

1. AVFormatContext (FormatContext)
2. AVChapter (Chapters support #15)
3. AVProgram (Programs support #16)
4. AVStream (Stream2)
5. AVFrame (VideoFrame2, AudioSamples2, av_frame_get_metadata(), av_frame_copy_props())

Note, that metadata of the FormatContext and Stream2 can be changed during playback. Refer to the FormatContext::eventFlags() and Stream2::eventFlags() family methods.