Add image sequence support to Pitivi

Blog: http://cfoch-dev.tumblr.com

The first goal is to get Pitivi be able to insert a "Image Sequence" clip. Then the user can edit this clip.

• GstImageSequenceSrc: https://github.com/cfoch/gst-plugins-bad/tree/sequences/gst/sequences

• Pitivi Image Sequence: https://github.com/cfoch/pitivi-cfoch/tree/sequences

Complete TODO List: https://ptv.titanpad.com/18