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.

headers/private/shared/AboutWindow.h

@@ -18,7 +18,6 @@
 class AboutView;
 class BBitmap;
 class BPoint;
-class BHandler;
 
 class BAboutWindow : public BWindow {
  public:
@@ -26,7 +25,6 @@ class BAboutWindow : public BWindow {
 								const char* signature);
 	virtual					~BAboutWindow();
 
-	virtual	bool			QuitRequested();
 	virtual	void			Show();
 
 			BPoint			AboutPosition(float width, float height);
@@ -48,9 +46,12 @@ class BAboutWindow : public BWindow {
 			const char*		Version();
 			void			SetVersion(const char* version);
 
+	static	BAboutWindow*	GetWindow(const char* appName,
+								const char* signature, bool* needsInit = NULL);
  private:
 			AboutView*		fAboutView;
-			BHandler*		fCaller;
+
+	static	BAboutWindow*	sAboutWindow;
 };
 
 #endif	// B_ABOUT_WINDOW_H