Visible Component Extensions

Currently, extensions are limited to just non-visible components. Due to this, extensions can't be dragged into the screen designer. To work around this, extension developers have to get a reference to a visible component as parent under which they create UI views. This, however, defeats the idea of App Inventor, which allows the users to create their UI of their apps using the concept of WYSIWYG. My proposal to solve this problem is to have a generic MockVisibleComponentExtension which can be dragged into the designer. The extension will be just a visible component as opposed to non-visible component. This allows the extension to be placed under a ComponentContainer under which the extension can create its views. This allows the users to view a clear hierarchy of their app's UI when using "visible" extension. Then, there will also be a need for MockComponentContainerExtension into which any visible component/extension can be placed.