Updated French translation
check: Present a human-readable error when gst-python is not working properly

This is 1000x better than the cryptic traceback users get when pygst is missing
check: Move dependencies to a single location and document for packagers
PEP8 compliance fixes in settings.py and render.py
check: Clean up the soft dependencies constants mess

This file is meant to make the packagers' job easier,
so it must be as legible as possible.
Releasing Pitivi 0.94 "Tricks or Tracebacks?"