Add perlSplit() overload taking a QRegularExpression and deprecate the QRegExp one

Summary:
This adds a perlSplit() overload that takes a QRegularExpression
parameter, and also deprecates the version that took a QRegExp param.

For both functions, return early if the string to be split is empty.

Test Plan: make && ctest

Reviewers: Frameworks, mpyne, dfaure, apol

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26657