Use fast seek in the media files previewer
Submitted by Jeff F.T.
I'm suspecting the seeking code in mediafilespreviewer.py (the module used for the file chooser dialog and the media library preview feature) is quite basic, in the sense that it always does accurate seeks.
To speed things up, we should leverage the new seeking types in GStreamer 1.x to scrub more quickly/efficiently while dragging the slider and do the final seek (when the mouse button is released) with an accurate seek.
Imported from https://bugzilla.gnome.org/show_bug.cgi?id=695262