Turn CuraApplication::_onExit into public api as saveSettings
Turn CuraApplication::_onExit into public api as saveSettings

Since we need it for the autosave plugin

Contributes to CURA-1615

Curahelper/Cura,totalretribution/Cura,hmflash/Cura,senttech/Cura,hmflash/Cura,senttech/Cura,totalretribution/Cura,fieldOfView/Cura,ynotstartups/Wanhao,ynotstartups/Wanhao,fieldOfView/Cura,Curahelper/Cura