Properly display secret chat hashkey

[BenWiederhake]

Currently, the user needs to:

• Magically know that she's supposed to talk about the secret hash key and make sure both users see the same
• Right-click on conversation, click on "Info", look at image
• Somehow "verify" it by talking about it, which is bullshit.

We should achieve at least this:

• Whenever a secret chat is opened, explain what's happening, and what the user needs to do. Let Logs of secret chats shouldn't be saved to disk #196 piggy-back here.
• Display the secret chat hashkey always at the top.
• Explain to the user needs to talk about it, since this must not be done in an automated way.
• Still require the user to somehow "verify" it by talking about it, which is bullshit; but we can't avoid it.

[BenWiederhake]

If anyone feels like implementing this enhancement, it might be a better idea to direct your energy to tdlib-purple. For now, there seems to be nobody who implements any enhancements for telegram-purple, so I'm closing all "enhancement" issues. (Aww man I hate closing my own issues.)
Of course, Pull Requests are still very much welcome!

(Note that tdlib-purple doesn't implement secret chats yet.)