Use Q_EMIT and build with QT_NO_KEYWORDS
Summary: QT_NO_KEYWORDS will be default in Qt 6 so all occurrences of emit should be replaced by Q_EMIT.
Test Plan: Build with QT_NO_KEYWORDS define enabled and observe succesful build.
Reviewers: kossebau, dfaure, apol
Reviewed By: dfaure, apol
Subscribers: ahmadsamir, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D29003