Add hidden option to toggle offline updates

[leolost2605]

Not sure whether this will be accepted but it adds a hidden setting to toggle offline updates, i.e. whether packagekits offline update mechanism is used and therefore a restart being required.
I think updates without restarting aren't the most dangerous thing and someone who manages to toggle this setting via dconf probably knows the risks.
My main reason for this is because I dual boot and have to spam keys during startup so every update forces me to sit in front of my pc for two minutes doing nothing and even though that's obviously not the worst thing ever I just cant be bothered anymore :)

[leolost2605]

And @elementary/ux because I can't request review here :)

[danirabbit]

I'm not sure about this. If you really want to update online you can do it from Terminal no?

Regarding your original problem though, I wonder if there's something special we need to do to use soft reboots? Supposedly systemd 254 has a mechanism for doing a "soft reboot" for offline updates that isn't actually a full reboot https://github.com/systemd/systemd/releases

[leolost2605]

If you really want to update online you can do it from Terminal no?

Yeah I'm 100% gonna forget to do that :)

But I already thought this is where this is going so closing :)