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>