TopComponent - the class for tabs and windows

We now have a module that is mostly empty. NetBeans created a few artifacts for us, but we need not concern ourselves with those, as the build will manage those for us. What we do need to do, though, is create our first GUI element, which will be something that NetBeans calls a TopComponent. From the NetBeans Javadoc, found at http://bits.netbeans.org/8.2/javadoc/, we find this definition:

Embeddable visual component to be displayed in NetBeans. This is the basic unit of display--windows should not be created directly, but rather use this class. A top component may correspond to a single window, but may also be a tab (e.g.) in a window. It may be docked or undocked, have selected nodes, supply actions, etc.

As we'll see, this class is the main component of a NetBeans RCP application. It will hold and control various related user interface elements. It is, to put it another way, at the top of a component hierarchy in the user interface. To create...