[style] Reparent window shadows to widgets

We want to destroy a shadow after its decorated window has been destroyed
or unmapped. In order to do that, we install an event filter on the widget
and make the shadow a child of the window.

Unfortunately, the underlying window and the widget have different life
times. To counter for that, we clean up _shadows after the shadow has
been destroyed. This turned out to be a bad idea because when someone
does qDeleteAll(_shadows), QMap iterators will become invalid.

Since KWindowShadow handles the case where the native resources for the
window have been released well, we can keep shadows as children of
widgets. This will simplify object hierarchy and fix invalid iterators
when ShadowHelper destroys all shadows with qDeleteAll().

Still, it would be nice to call KWindowShadow::destroy() after the native
resources for the window have been released.