Improve behavior of the app if we failed to save a copy of already sent message

[DenBond7]

We can have a situation when IMAP server provides Sent label with wrong or missed attributes. Currently, we identify the Sent folder by "\\Sent" attribute. If we couldn't identify the Sent folder we are not able to save a copy of an already sent message on a server.

[DenBond7]

It'll be great to show something like this

Then a user will be able to specify the Sent folder.

[DenBond7]

I've added some changes which will resolve some cases like "INBOX\Sent", "Sent". And a user will able to notify us if he has any problem with saving a copy in the Sent folder.

For now, it's enough. I've created a new issue which should improve user experience. I'm closing this issue.