asfdemux: if video is h264, check the codec_data for bytestream data

For bytestream we don't want to expose it as codec_data but rather as
streamheader as it is not out-of-band data but data that should be
prepended to the beginning of the stream before the other buffers.

https://bugzilla.gnome.org/show_bug.cgi?id=735070
asfdemux: add ASF_OBJ_METADATA_LIBRARY_OBJECT

We can safely ignore those but best to claim to understand this object type so
we don't raise a warning.

https://bugzilla.gnome.org/show_bug.cgi?id=736060
asfdemux: first try to delegate TIME seek events upstream

We should give a chance to upstream to handle TIME seek events before trying
to handle it in asfdemux.

Fix a bug where fast backward seeks where discarded right away because they
were not supported by asfdemux.

https://bugzilla.gnome.org/show_bug.cgi?id=734471