multifilesrc: implement uri handler
multifilesrc: implement uri handler

With this patch we can now provide a set of files
created by multifilesink as a source for uri elements.

e.g. gst-launch-1.0 playbin uri=multifile://img%25d.ppm

Note that for the %d pattern you need to replace % with %25.
This is to be compliant with URL naming standards.

https://bugzilla.gnome.org/show_bug.cgi?id=783581

StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good