Base class for all elements, handles layout and styles. Instances of this class have no content and no children.

Elements with content (and no children) derive from ContentElement.

Elements with children derive from BasePanel.

This class doesn't do much by itself except call virtual methods that can be overridden in derived classes. It can draw a background and borders with no content, so it can be used to implement things like bars and spacers.

It is designed to be very light-weight, since typical documents may create thousands of these. Additional elements should be added to not-so-common derived classes.