Set "typing" status

[gnprice]

I.e., half of this feature:
https://zulip.com/help/typing-notifications
along with #665.

In the API, the relevant endpoint is:
https://zulip.com/api/set-typing-status

For an implementation for comparison, see zulip-mobile's sendTypingStart and sendTypingStop, or the corresponding code in Zulip web.

[IfeanyiSam]

Is this issue open to be worked on?

[chrisbobbe]

No, #897 is already in progress for this issue. Please choose a different one.