splitmuxsrc, splitmuxsink: lazy loading and dynamic fragment addition

This MR adds some new abilities to splitmuxsink and splitmuxsrc, aimed at improving support for recordings with a large number of files, and for playing back ongoing recordings.

The overview of the changes is:

• You can now add fragments directly to splitmuxsrc and provide the offset and duration in the stream.
  • Providing offset+duration means splitmuxsrc doesn't need to scan the file to measure it and calculate it. That makes for much faster startup
  • The new add-fragment signal can be used to add files to the set dynamically - allowing to be playing an ongoing recording and adding files to the playback set as they are finished.
• splitmuxsrc no longer keeps all files open, but instead only keeps 100 open by default, configurable with a property
• There is a lookahead property on splitmuxsrc to trigger (re)opening files a certain distance ahead of the play position
• splitmuxsink will report fragment offset and duration when closing a file

There are also some refactor / bug fix commits that would be slightly annoying to break out of the patch stack now.