Add a chat command for disabling screen sharing

[ian-kelling]

A screen reader user asked me how to stop sharing their screen in Galene, and I said "hover your mouse over it and press the x", and they said hovering was not something they could do.

A chat command would work for them.

[jech]

Sure, makes sense. I like chat commands too, I find them much easier to use than buttons. Please ask the user if there are any other chat commands they would find helpful.

[jech]

Done in e59880e.