Implement button actions for staff login dialog.

[maillouxc]

The cancel button should close the dialog. (If I remember right, I think the scene builder has a specific cancel button property, but I may easily be wrong.)

The login button should, for now, fetch the text fields from the button (don't need to do anything with them, just grab them from the text boxes and put em in local variables inside the button click method in the controler) and then just show the staff home screen. We don't need to worry about implementing actual authentication yet.