Stop query date regex from matching > 4 digit year

[afontenot]

Some queries are ambiguous between dates and numbers, e.g. "2002" could be a year or a number. In order to simplify quering parsing, Quod Libet stores these ambiguous subqueries as the "NumexprNumberOrDate" type.

When parsing a query, Quod Libet will use this type if the string matches a regex for parsing dates. However, before this commit, the regex will match the first four digits of any four or more digit number. This results in a parsing error for large numbers because of the remaining digits after the four that get matched and parsed.

Instead, use a negative lookahead to limit the year part of the date to four digits. Quod Libet will fall back to using an unambiguous number representation for large numbers.

As an alternative, I tried to allow >4 digit years for future-proofing, but a 4 digit year assumption is baked in too deeply, e.g. in Python's strptime function.

Fixes #4161.

Check-list

• There is a linked issue discussing the motivations for this feature or bugfix
• Unit tests have been added where possible
• I've added / updated documentation for any user-facing features.
• Performance seems to be comparable or better than current master

[declension]

Thanks