StopTasksDialog: don't regenerate _cancelButton
StopTasksDialog: don't regenerate _cancelButton

This was made possible by a0ce58ccb4b9b374d66883c3b68b762e1ea06858.

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