Improve UX for sending friend request

[dvor]

Sometimes users don't have full Tox ID and are confused by send button being not available. It would be nice to add "characters left" counter, maybe enable send button and improve error messages. Also some description about Tox ID and where to get it can be added.

[katcom]

Maybe there can be a dialog requiring the nospam code , also can be known as the security number, if the app spotted ToxID that only was in the part of public key.