Feedback when sending messages

[JulianGro]

Currently, when you send a message, the dialogue just closes instantly. 

Personally, this often makes me initially think that the app has crashed.

My uncle also got confused by there being no confirmation at all that Thunderbird is trying to send the message to the point that he sent the same email like 10 times and then asking me if I can check why the email program isn't working.

It would be great to have a little animation of a paper airplane flying away, or a little popup saying "Message sent". 

[Jon]

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