baseparse: remove is_seekable vmethod and use a set_seek instead

baseparse: remove is_seekable vmethod and use a set_seek instead

Seekability, like duration, etc is unlikely to change (frequently), and
the default assumption covers most cases, so let subclass set when needed.
At the same time, allow subclass to indicate if it has seek-metadata (table)
available, and possibly have it provide an average bitrate.