qtmux: handle issues correctly when downstream is not seekable

qtmux: handle issues correctly when downstream is not seekable

The streamable property only make sense for fragmented formats.
For regular MP4, when downstream is not seekable we can't rewrite
the headers, so qtmux can only work with fast-start=TRUE, where
the headers are written finishing the file.
For fragmented MP4, when streamable is not seekable and the streamable
property is FALSE, we must enforce streamable=TRUE warning the user
about this change

https://bugzilla.gnome.org/show_bug.cgi?id=707242