logout-dialog: New Xfconf property "ShowSave"
The new ShowSave property is under general and defaults to TRUE. /shutdown/ShowSave was also another option but I didn't find it fitting.
Personally, I never use autosave and never save on logout. I find the save session checkbox infuriating/annoying/distracting, hence I decided to take this matter in my own hands. 
Edited  by Akbarkhon Variskhanov
