We would like to have more user-choices about what to do with the pending tasks 
and these choices could be persisted if users want to.  

 Among the choices are:  
1. cancel exit, 
2. exit with termination of selected tasks but leave the unseletected running
3. same as 2, but persisting user choice on each task and do not ask again about 
them next time user exit.  

On persistence of choices:
- Next time users exit, ExitDialog should check the pending tasks against the 
known choices, if there are no tasks with unknown choice, the pending task should 
not be brought up and user choice on the pending tasks should be performed 
(shutdown or leave running).
- This option setting should be resettable through the option menu to default 
(always ask users about all running tasks)