[kstyle] Port to QWindow::startSystemMove()
Summary:
QWindow::startSystemMove() is a new feature in Qt 5.15 that provides a
platform-independent way to start an interactive move operation.
WARNING: I have no idea whether KDE Plasma 5.19 is going to depend on Qt 5.15.
Reviewers: KWin, Plasma, hpereiradacosta
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27003