common: Port IgnoreListManager to ExpressionMatch

common: Port IgnoreListManager to ExpressionMatch

Port IgnoreListItem objects to ExpressionMatch class, providing easy
caching and simplifying expression handling.

Migrate IgnoreListItem struct into a full-blown class for easier
management and greater assurance over automatic internal cache
management.

Add tons of documentation comments, too, and fix up line lengths.

Thanks to @sandsmark for the initial efforts towards the
QRegularExpression migration; it helped a lot!