Fix BAboutWindow lifecycle

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.