xls: when writing strings that cannot be represented, substitute a ?
xls: cleanup.
xls: reverse ("aaa"&""bbb"&...) hack for long strings when we read

This way, formulae with string constants longer than 255 characters will
appear to save perfectly fine in xls format.
Tests: add string tests for the Greek alphabet.

These characters aren't representable in the default biff7 codepage,
so we expect to see "?" instead.
Tests: remove WINDOWS_LANGUAGE from environment.

libgsf listens for that and it might affect the codepage it picks
for certain things.
Tests: remove LANGUAGE too.

"locale" claims it makes a difference.
xls: simplify image reading.

Handling BIFF_CONTINUE in ms-biff.c makes for much more
readable code.
xls: go wild with BIFF_CONTINUE handling.

This covers everything that I can find that appears to have potentially
large sections in them: formulae, strings, images.