mp4dashmux: add the tfdt atom to the moof

This include the basedecodetime for each fragment needed
for seeks in the demuxer
qtmux: deprecate the streamable property for non-fragmented MP4

The streamable property only makes sense for fragmented MP4.
https://bugzilla.gnome.org/show_bug.cgi?id=707242
qtmux: make "streamable" TRUE as default

The most common use case for fragmented MP4 (Dash and Smooth Streaming)
is producing streamable content (even for VOD). streamable=FALSE would only
be used to generate fragmented MP4 with and index of MOOF's that could
be reproduced without a playlist/manifest
https://bugzilla.gnome.org/show_bug.cgi?id=707242
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