Create an event/callback on `kivy.core.window` so accessibility implementations can know when it's the right moment to attach to Window

[misl6]

As explained by one of the AccessKit core developers (and as shown by a POC), in certain cases, like when running on Windows, accessibility tools need to "install" (attach on the native window, to capture the messages) BEFORE the Window is shown.

The proposal, to avoid doing any hacky (and ugly) "patching" is to add a new event/callback to notify the accessibility provider that should attach to the Window.

Before implementing it, that needs discussion.