Use the LyX name of encodings instead of the LaTeX names.

Use the LyX name of encodings instead of the LaTeX names.

- Use the LyX name of encodings instead of the LaTeX names.
  The LyX name must be unique, while the name used by LaTeX
  not necessarily, e.g. different packages might implement
  support for the same encoding.

- Rename koi8 to koi8-r, so that the LyX and LaTeX names match.

- Rename euc-jp-plain to euc-jp-platex, jis-plain to jis-platex
  and shift-jis-plain to shift-jis-platex.

- Add utf8-platex encoding (fixes #8408).

LyX file format incremented to 463.