[Request]: Autocomplete known user handles when writing posts

[N07070]

Describe the request

Hello ! 👋

Thanks a lot for forking this software, I find it very useful and would like to contribute to it when time permits 🙂

I would like to add the ability to autocomplete the known user handles when you start typing @, very much like it works on web or other clients.

This would indeed make it simpler to write posts and interact, by reducing user efforts.
Cheers;

Implementation Details

• This should be an option in settings.
• This should be only available to some fediverse backends. (Include which ones on the above field).
• This is client-only (and shouldn't sync with the instance).
• This follows the GNOME HIG.

[GeopJr]

Thanks for raising this issue! It should be done as of #137