Q/Date/Time: align unquoting format strings for both from/toString()

All Q/Date/Time::fromString/toString() methods should handle single-quotes the same way, so that a format string survives the round-trip of fromString()/toString(). There are currently no known bugs because of this, but the parsing should be uniform anyway. At present the docs aren't even consistent with one another; and the behaviour doesn't seem to match the intended (CLDR-compatible) treatment of two adjacent single quotes in the format string (regardless of whether within other single quotes) as denoting a plain single quote character in the date/time string.

The CLDR specification gives, as one of the forms of verbatim text:

Two adjacent single vertical quotes (''), which represent a literal single quote, either inside or outside quoted text.