VisualBuilder: Use real GWidgets instead of pretend VBWidgets.

That first design was the wrong idea. Instead, have VBWidget instantiate
a GWidget of the appropriate type and parent it to the VBForm.
We then use a new "greedy hit-testing" mechanism in GWidget to prevent any
mouse events from reaching the VBForm's children.

To paint the grabbers above the child widgets, I added a slightly hackish
but kind of neat second_paint_event() that is called after a widget has
painted all of his children. :^)

Applications/VisualBuilder/main.cpp

@@ -46,7 +46,7 @@ int main(int argc, char** argv)
     app.set_menubar(move(menubar));
 
     auto* window = new GWindow;
-    window->set_title("Form1");
+    window->set_title(form1->name());
     window->set_rect(20, 200, 640, 400);
     window->set_main_widget(form1);
     window->set_should_exit_event_loop_on_close(true);