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.

src/apps/activitymonitor/ActivityView.cpp

@@ -600,10 +600,6 @@ ActivityView::~ActivityView()
 {
 	delete fOffscreen;
 	delete fSystemInfoHandler;
-
-	// replicant deleted, destroy the about window
-	if (fAboutWindow != NULL && fAboutWindow->Lock())
-		fAboutWindow->Quit();
 }
 
 
@@ -647,8 +643,6 @@ ActivityView::_Init(const BMessage* settings)
 	const char* name;
 	for (int32 i = 0; settings->FindString("source", i, &name) == B_OK; i++)
 		AddDataSource(DataSource::FindSource(name), settings);
-
-	fAboutWindow = NULL;
 }
 
 
@@ -1112,22 +1106,26 @@ ActivityView::MessageReceived(BMessage* message)
 
 	switch (message->what) {
 		case B_ABOUT_REQUESTED:
-			if (fAboutWindow == NULL) {
+		{
+			bool needsInit;
+			BAboutWindow* window = BAboutWindow::GetWindow(kAppName, kSignature,
+				&needsInit);
+			if (needsInit) {
 				const char* authors[] = {
 					"Axel Dörfler",
 					NULL
 				};
 
-				fAboutWindow = new BAboutWindow(kAppName, kSignature);
-				fAboutWindow->AddCopyright(2008, "Haiku, Inc.");
-				fAboutWindow->AddAuthors(authors);
-				fAboutWindow->Show();
-			} else if (fAboutWindow->IsHidden())
-				fAboutWindow->Show();
-			else
-				fAboutWindow->Activate();
+				window->AddCopyright(2008, "Haiku, Inc.");
+				window->AddAuthors(authors);
+			}
+
+			if (window->IsHidden())
+				window->Show();
+			window->Activate();
 
 			break;
+		}
 
 		case kMsgUpdateResolution:
 		{