Your limited to the things panel supports when your using a PANEL* but you can also use API's look in the manual or you can manually create interface items. Believe it or not programming actually requires you to be creative commands are not always just laid out for you to create everything sometimes you have to make it yourself.
im all for that but making your own gui? seriously thats like reinventing the (not so easy to do) wheel
i want to work with the tools not make tools because what would be the point of engines in the first place...