Summary:
Ready for QJSEngine port and upcoming other fixes.
Split as it makes it easier to do any before/after testing.
Test Plan:
All tests pass with the current QScriptEngine
Verified expected API against a wiki page and current code.
Reviewers: #kwin
Subscribers: kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D14482
