BAboutWindow returned false in QuitRequested in order to hide instead of closing.
Not only this keeps a BLooper running for a rarely used window, but it also
prevents quitting an application in the window was not destroyed first.
* Remove aforementioned QuitRequested method,
* Add a static GetWindow method that returns the existing about window, if there
is one, or creates one if there is not. A boolean can be set to tell the caller
what happened,
* Adjust all callers to use that new method, instead of managing the window themselves.
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