pitivi: 0.94 -> 0.95

Notable (hard) dependency changes:
  - gstreamer >= 1.6.0
  - gst-plugins-bad with GTK support (need "gstgtk" plugin)
  - pygobject >= 3.14
  - gst-validate
  - matplotlib (used for drawing the "timeline")

Changes v1 -> v2:
pitivi no longer requires clutter, so I switched clutter-gtk => gtk3.