Add support for resending failed sends

[Jacalz]

Checklist

• [X] I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• [X] This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

It can be annoying to find the file, folder or text again if something fails. Adding a button to resend would make things easier.

Describe the solution you'd like to see.

Allow an option to press resend if it failed.

[ihubgit]

and even more so a button to resume where the download left off...

[Jacalz]

and even more so a button to resume where the download left off...

Resuming will be very complicated and might be problematic. We know how much we have sent, but do we know how much has been received (i.e. are they always the same if we get an error)? If we then restart, we need to only read new data from the file (requires io.ReadSeeker) and also open the receiving side without truncation. The code will become very complex. It might be doable but a full restart is better than nothing.