Separation of the various names for encodings.

Separation of the various names for encodings.

Provide functions for translating to the LyX name
of an encoding from either a LaTeX name or an Iconv
name, with the possibility to specify the package.
This is in anticipation of changing to use the LyX
name of the encoding in the .lyx file format and
allowing multiple lib/encodings entries to have
the same LaTeX name (but different packages!).

The tex2lyx parser needs to worry about the iconv
name of the input encoding, so store that instead
of the latex name.