Application: extract the auto exit behavior into a separate method from __removeWindow()

This will allow OpenFL to override the behavior based on the NativeApplication class.

src/lime/app/Application.hx

@@ -549,12 +549,17 @@ class Application extends Module
 			__windowByID.remove(window.id);
 			window.close();
 
-			if (__windows.length == 0)
+			__checkForAllWindowsClosed();
-			{
+		}
-				#if !lime_doc_gen
+	}
-				System.exit(0);
+
-				#end
+	@:noCompletion private function __checkForAllWindowsClosed():Void
-			}
+	{
+		if (__windows.length == 0)
+		{
+			#if !lime_doc_gen
+			System.exit(0);
+			#end
 		}
 	}