Distinguish all spaces for QTextOption::ShowTabsAndSpaces

When QTextOption::ShowTabsAndSpaces is used, we currently (since QTBUG-57479) distinguish:

• space as a dot
• tab as an arrow
• non-breaking space as a degree sign

There are diverse other characters that Unicode classifies as space, that it would make sense to mark up similarly; for example, the thin space U+2009 and its non-breaking partner U+202F. We can query relevant properties from QChar using its category() == Separator_Space and decompositionTag() == NoBreak tests.
(For the latter, see here.)