Patch by Anthony Loiseau (thannoy _AT_ actech-innovation _DOT_ com)

 Always reject plugins that have a 0 priority but did not match the
shortcut list (being empty or not).
This makes 0 priority plugin only selectable using a shortcut (which I
think it is what we want).

avcodec will now output extradata using the same format than flac encoder.

Previously, it was possible only on the first subtitle.
The drawback is that we may allow selecting channels that are empty.

since we may want sub-second granularity.

That plus the fact that the decoder already limit its fifo will allow
to mark all --*-cache options safe.

It will prevent #2706 from broken demuxer/streams.

It closes #2706.

The sequence header/entry point are not repeated yet.

Modified and Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

The new layout is a horizontal layout, for people having a smaller screen than
1000 in height. This makes this dialog quite large, but screens are larger those days. If the dialog is still too big, please say so.

This is easier to understand and you can still modify the MRL.

There are some risks of regression (in case of typo), but it was
a nightmare to maintain.
Also by using the right model (h264.c) it fixes some timing issues in
vc1 and mpeg2 packetizer.