[style] Fix crash during app tear down sequence

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 ensure that _shadows has no dangling pointers, we will remove the shadow when it's destroyed. The problem with doing so is that it will invalidate iterators when ShadowHelper destroys all shadows during app tear down, which will eventually result in a crash.

In order to keep iterators valid when ShadowHelper destroys shadows, we need to clean up _shadows after the decorated window has been destroyed.