multifile: add support for reading filenames from a list file

Submitted by Jonathan Matthew

Link to original bug (#615166)

Description

Created attachment 158188
listfilesrc element

After watching someone on IRC struggling with multifilesrc for the nth time, I figured it'd be easier to use in some cases if you could give it a file containing a list of source files, rather than having to number the files sequentially.

What I'm attaching is a quick hack based on multifilesrc. It might be worth considering adding the functionality to multifilesrc rather than creating a new element, or sharing the common code (which is most of it) some other way.

Attachment 158188, "listfilesrc element":
gstlistfilesrc.c