8956 cmx transitions (#1)

Revisited how transitions are handled by the cmx 3600 adapter.

- Transitions lines are now collected when parsing EDL lines and treated by the ClipHandler.
- The clip source and record points are replaced with the transition values when the line is parsed.
- The transition in_offset is now 0 and the out_offset is the transition duration, matching in the timeline what would be done if the editing was read from another format.
- The transition is added just before the clip when it is added to a track, and that's it.
- Adjusted tests for the new way of handling transitions, added some other checks.
- Added an EDL read/write/read roundtrip to check that the reader and writer are compatible.
- Removed code which was not needed anymore.