Sadly you cannot do a widget within a widget with MVC, same for Sections within Sections.
In your case, there are some options. You can use Page Relationships to Select multiple buttons for your Banners which would be also a page type, similar to how Roman is describing. You still need to 'create' these buttons as individual items so you can select them, which i know isn't 100% ideal.
A second option (Probably best) is to make the Banners individual pages on the content tree, and enable Page Builder on them, so you can add multiple widgets to your "banner" that way. I have a tool that you can then pull in the page and render it's widgets, called a Partial Widget Page. We use this so users can have sections and widgets in Mega Menus and pull them into the header.
Another option which is probably not ideal is you could have the buttons as inline widgets (kind of portal-engine like) and use Lee's MVC Widget Resolver to add buttons as inline widgets on a rich text field on your page type, then render that out, however be aware that this will probably be only a "1 version" thing as next kentico version all portal engine widgets/webparts will be gone (although hopefully we'll have an inline mvc widget by then).
Check out the Partial Widget Page tool, i think it will do what you are looking to!