QtQuick.Dialogs: on single-window platforms, don't resize the window

m_dialogWindow only needs to be set if the dialog has its own window,
because otherwise setters such as QQuickAbstractDialog::setHeight
(introduced in 37492efee021ccc6e550ecb67a3240b76eb2219e)
will result in reshaping the main application window instead of the
dialog.  Also the window decoration's translucent "veil" should
expand to fit the whole application.

Change-Id: I16fdb48c54dbc5e758dfec1f18063579ac2bbb05
Reviewed-by: Liang Qi <liang.qi@digia.com>