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).