Receiving text with an already open receive window should ask to overwrite

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• 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?

Receive some text and keep the window open. Receiving more text will overwrite the window contents without asking to do so. We ought to at least ask the user if they want to keep the old one or dismiss the new one.

Describe the solution you'd like to see.

See above