qtmux: add AC-3 muxing support

qtmux: add AC-3 muxing support

Adds AC-3 muxing support. It is defined for mp4 and 3gp formats.

One extra feature that was added was the ability to add extension
atoms after set_caps as the AC-3 extension atom needs some data
that has to be extracted from the stream itself and is not
present on caps.