Various tag tweaks.

This PR replaces #250. In individual commits are:

1. Resequence tag list in id3.py to match sequence in http://id3.org/id3v2.4.0-frames to assist with audit of what we cover / don't cover.

2. Move work tag from TXXX:Work to TOAL (and provide translation for anyone who has used the daily build since the earlier PR created the Work tag).

3. In line with other TXXX tags, save the Writer tag as Title Case (and provide translation for anyone who has used the daily build since the earlier PR created the Writer tag).

4. All checks for "lyrics" are of the form "startswith('lyrics:') or = 'lyrics' " except one - so fix this one. (Important since a previous PR of mine allows lyrics without a ':'.)

5. Add support (in ID3, ASF, and APEv2) for Encoder Settings (TSSE tag in id3v23/4).

6. Meet the id3v24 specification by setting the TDTG tagging-timestamp tag when we save the file.

7. Remove obsolete tag names from the list of tags.

8. Improve tag descriptions for Work -> Work Title and Website -> Artist Website.

9. Avoid possibility of duplicate MBIDs in a tag.

Note: Discid and xml error checks which were in #250 are not in this PR.