ges-launch: replace home-grown version of gst_filename_to_uri()
ges-launch: replace home-grown version of gst_filename_to_uri()

and remove superfluous check if file is readable with
fopen. Code appears to also want to accept URIs, so this
doesn't work so well, and should probably be done differently
anyway if required.

https://bugzilla.gnome.org/show_bug.cgi?id=674296

cfoch/ges,volodymyrrudyi/gst-editing-services,cfoch/gst-editing-services,pitivi/gst-editing-services,cfoch/gst-editing-services,pitivi/gst-editing-services,cfoch/ges,cfoch/ges,pitivi/gst-editing-services,cfoch/gst-editing-services,pitivi/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,volodymyrrudyi/gst-editing-services,volodymyrrudyi/gst-editing-services