Validate register page when choice is changed.
Validate register page when choice is changed.

Also setFocus() for nextButton so that the user can proceed only by pressing Return key.

shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,shinnok/tarsnap-gui,shinnok/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui