common: Make regular expressions Unicode-aware

common: Make regular expressions Unicode-aware

Without the UseUnicodePropertiesOption flag, Qt's regular expressions do
not include extra-ASCII codepoints when matching character classes. This
results in \W (used by the highlight logic to prevent matching against
substrings) matching against letters with diacritics, meaning the words
'Västra' and 'TÜV' would both count as a highlight for the nick V.

Add unit tests to verify this functionality is correct in Quassel and
any projects that implement ExpressionMatch.

Co-authored-by: V <v@anomalous.eu>