feat(core): use new SpinnerWidget of ubo-gui to show unknown progress in notifications, and add General sub menu to System settings menu to host ubo-pod/ubo-app related settings, currently it has Debug toggle to control a debug feature of HeadlessWidget - closes #190

feat(core): use new SpinnerWidget of ubo-gui to show unknown progress in notifications, and add General sub menu to System settings menu to host ubo-pod/ubo-app related settings, currently it has Debug toggle to control a debug feature of HeadlessWidget - closes #190 #833