Replace Customers & Employees classes with Lists.
Replace Customers & Employees classes with Lists.

This was meant to be included in previous commit.
Provide one location to update PeopleListBox.

Currently the mechanism to update ListBoxes (by setting to null first)
is hackish and causes selection to be lost and should be fixed, however
it works for the time being.
Add error handling to static Add method.

Add method now returns bool. True if successful, otherwise false.