Kdenlive linear timewarp (#869)

* General improvements to kdenlive adapter made while trying to convert a .prproj using my otio-premiereproject adapter to .kdenlive and then opening in Kdenlive.

* reverted reording tracks on write (I'll have that done by my otio-premiereproject on read, if anywhere). Reverted offsetting all time to 0 (I think Kdenlive either does or should handle that).

* [kdenlive adapter] converting RationalTime to/from mlt time string now matches algorithm from mlt source code (haven't properly looked into it, but it's possible Python's str(datetime.timedelta(...)) is broken). Correctly uses end_time_inclusive when writing time ranges - and equivalent when reading. Converts file:/// uris to file path on the way out.

* [kdenlive] fixed for Python 2

* [kdenlive] added a test using the same FCP xml I was using for my own testing. An export from Premiere from a genuine thing I was editing. In the process of writing the test discovered another bug in the from Kdenlive time string function. Written it to accurately reverse the process in the clock function; written having referred to the source code of mlt. That source code is technically flawed. I believe it isn't written with non-integer frame rates in mind.

* kdenlive writing handles nested references to other kdenlive files.

* [kdenlive linear timewarp] Supports multiple LinearTimewarp effects on a clip when writing Kdenlive.

* [kdenlive linear timewarp] fixed a few bugs related to generators and missing references introduced by previous changes.

Author: splidje