viewer: Do not forget to set ViewerWidget sink when setting our pipeline

Making the expose call working when we need to redraw the viewer.

+ Rename Pipeline._glimage_sink to Pipeline.videosink and making it
  public and simpler to read.

Conflicts:
	pitivi/mediafilespreviewer.py
check: Do no require GNonLin if GES version >= 1.5.0

As it will be directly integrated in GES itself