Deprecate and avoid unload event handler

[niloc132]

This is a proposal to resolve #9908 - the unload and beforeunload event handling is split now, so that only explicit calls to WindowImpl.initWindowCloseHandler(), Window.addCloseHandler(), or Window.addWindowCloseListener() can cause the unload event handler to be attached - all of which are now deprecated. The beforeunload event wiring is now the only event handler set up when Window.addWindowClosingHandler etc are called.

Core formerly intended to prevent memory leaks during the unload event is removed, since modern browsers do not leak these handlers.

Fixes #9908