Don't use the QRegExp methods that modify the object

QRegExp matching methods modify the object, which we don't want to. In
particular, when we receive a QRegExp from the user or we store in a
context that might require thread-safety, make sure we make a copy
before using it.

QRegularExpression has no such shortcoming.

Task-number: QTBUG-25064
Change-Id: I2c4d5f4b60d6b3569568103cd7107cd2adaa3ae8
Reviewed-by: Giuseppe D'Angelo <dangelog@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>