Improve unicode to ascii ...

for punctuation, normalization (merging similar looking characters into
the most common one), accents, and full conversion to ascii.

Functions and tests moved into separate files.

Although more comprehensive, the code should run faster because it
eliminates several loops (inc. a loop with two unicodedata references).

This is intended to form the basis of future PRs to:
a. Clean up (simplify) the file naming code
b. Provide script function(s) for cleaning individual tags / file name
parts
c. Add support for translation / transliteration plugins (which I think
make more sense than being included in Picard itself).
d. Support for converting Tags to ISO-8859-1 rather than ascii (since
that is what is supported by ID3 at least)
e. Possible additional options for allowing / preventing normalization,
possible reorganisation of options to centralise all encoding settings
onto one page rather than metadata, tags and file naming pages at
present (to be discussed).
Remove unnecessary lines (zas).
Address comments by 96187 ...

... whose local knowledge of how characters are actually used identified
more appropriate characters to map to.

soft hyphen -> nothing (since it is invisible)
multiplication -> x rather than * (visually closer)
remove duplicate oe
Address various comments by Philipp Wolfer ...

... based on actual knowledge of how characters are used in practice.

low quotation mark -> normal quotation mark rather than comma
oi should be gha -> gh rather than oi
ideographic punctuation, multiplication / division and middle dot are not punctuation
Fix metadata to import from new location
Merge pull request #263 from Sophist-UK/sophist_improve_unicode_to_ascii

Improve unicode to ascii ...