SSA/ASS probing: be stricter in what's accepted

Otherwise the information might also be found in CodecPrivate in a
Matroska file.
mkvmerge: probe text formats before other formats

Rationale: text file probing is a lot faster and usually much more
certain of the result. It shouldn't have to be done after the uncertain
MP3/AC3/AAC probing.
mkvextract: display attachment IDs correctly
SSA/ASS reader: fix UU decoding; also refactoring of said code

First issue was that attachments whose length was divisible by 3 were
truncated by 3 bytes. For those that weren't the last bytes were
incorrect (usually 0xff).

Fixes #1003.