fix notification of QDockWidget when it gets undocked

fix notification of QDockWidget when it gets undocked

Origin: upstream, https://code.qt.io/cgit/qt/qtbase.git/commit/?id=06b8644953fc5267
Last-Update: 2019-06-14

Before the patch the notification was emitted only when the docker
was attached to the panel or changed a position on it.

It looks like the old behavior was documented in a unittest,
so this patch might actually be a "behavior change".

Note: https://mail.kde.org/pipermail/distributions/2019-April/000317.html

Gbp-Pq: Name qdockwidget_undocked_notification.diff