Add a window to the top-level component

This means that we always have a window and for the C++
API we don't require this ComponentWindow use anymore.

examples/cpptest/main.cpp

@@ -15,6 +15,5 @@ int main()
         component.set_counter(component.get_counter() - 1);
     });
 
-    sixtyfps::ComponentWindow window;
-    window.run(&component);
+    component.run();
 }