Fix undo tests

That implies:
    * Use the pitivi Project  subclass of GESProject in the tests, that
      is needed for the undo subsystem to work properly (we need access
      to the pipeline to flush it)

    * Make sure that audio and video tracks are added to newly created
      Blank projects

    * We properly disconnect from TrackElements when removing a clip
      from a layer

    * Adding a clip leads to 3 Actions as GES will set the height and
      width of clips now