Add "Recent conversations" view

[varkor]

In Zulip Desktop, there is a "Recent conversations" view, which is very useful as it allows the user to find a conversation without scrolling through unrelated messages in other threads in "All messages". It would be very helpful to have such a view in the app too.

[gnprice]

Thanks for the feedback! This is indeed something we're planning to add in the future, in the upcoming Flutter-based Zulip app:

• Add Recent conversations view zulip-flutter#315

I'll leave this issue open as a pointer to that one, because it is a feature that's absent in this legacy app. But because this app is in maintenance mode, the place we'll ultimately add the feature is in the new app.