Add client flags to turn dnd on/off

[bvergnaud]

Hello 👋🏻

This is a QoL PR I offer, for which I'll be the first client. Unless I missed something, setting dnd state to on or off currently requires getting the current dnd state, and toggling if necessary. With this MR, it'll be doable in a single call to the client. Less bash scripts everywhere can only be a good thing for the world. 😛

Fair warning:

• This is my first time poking in this codebase. I might have missed things. Feel free to tell me if anything needs tweaking.
• Up until today, I'd never even heard of vala. The edit is small and simple, and the code seems pretty straightforward, but reviewers should assume it's been written by a newbie, I won't take it personally. 😄

Thanks !

[ErikReider]

Otherwise it looks good to me! :)

[ErikReider]

LGTM! Thanks for the help! :)