avformat/demux: Add new demux.h header

And move those stuff already in demuxer-only files to it.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/tests/seek_utils.c

@@ -18,7 +18,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-#include "libavformat/internal.h"
+#include "libavformat/demux.h"
 
 int main(void)
 {