multifilesink: add "aggregate-gops" property to process GOPs as a whole

multifilesink: add "aggregate-gops" property to process GOPs as a whole

This property can be used in combination with next-file=max-size
(and perhaps a future next-file=max-duration) to make sure that
each file part starts cleanly with a key frame and the appropriate headers.

In order for this property to work correctly, upstream elements should make
sure than any headers that need to be written in a standalone file are:
1) in the streamheader caps field
2) and/or in the stream as one or more buffers marked with GST_BUFFER_FLAG_HEADER
   that are just before the keyframe buffer

This is useful for MPEG-TS/MPEG-PS file segmenting in
combination with mpegtsmux or mpegpsmux.

Original patch by: Tim-Philipp Müller <tim@centricular.com>