[lipstick] Fix crash on close

When destroying LipstickCompositor ~QQuickWindow() is called after
~QWaylandQuickCompositor(), so changes to the items in the window may end
up calling code such as LipstickCompositorWindow::handleTouchCancel(),
which will try to use the compositor, at that point not usable anymore.
Merge pull request #213 from giucam/segfault_on_close

[lipstick] Fix crash on close