This method is used to perform functionality when the panel itself is
displayed. It is the responsibility of concrete implementations to set
the state of the GUI, such as the 'Next' button enabled/disabled. The
'Next' button is disabled by the GUI at the start of each 'Next' action.
Concrete implementations must re-enable it before this method returns.
For instance, you want to disable the 'Next' button while an intense
calculation is ongoing, then re-enable it.