Correct positioning for popups in QQuickWidget

Handle the case where the QQuickWindow is hidden and the content
is rendered somewhere else using QQuickRenderControl.

Task-number: QTBUG-38116
Change-Id: I51fb9001c307f53bf7d724860b2a66287982724e
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Add changelog for 5.3.0

Change-Id: I99fadbd5dff0a4ec5e6abd5b8a6b5b19cbfcace7
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Add Window declaration to plugins.qmltypes

QmlDesigner needs to know that Application window is derived from
QQuickWindow.

Task-number: QTBUG-38663
Change-Id: I4a90dc6a0d69ad5688431c86014cbdb36259caab
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Yuchen Deng <loaden@gmail.com>
Merge remote-tracking branch 'origin/release' into stable

Change-Id: I7d6be51a5601fb90ed5e102b63173ccc01b39f36
Merge "Merge remote-tracking branch 'origin/release' into stable" into refs/staging/stable