This now validates with
```
appstream-util validate app.briar.gtk.xml.in
```

This wasn't possible before because my version of flatpak was buggy.

Fixes #46.

Fixes #45.

Fixes #44

This reverts commit 2c5d71f0.

This is a change not-yet upstreamed but currently to be reviewed at
briar!1242.

Before, messages from all chats were added to the current chat.

Also scrolls to bottom now after adding a message async.

Before, there was no indication to which day a message belongs.

Before, more and more listeners got added to the chat entry, resulting
in the bug of message text not being visible after posting.

Fixes #25.

Fixes #6.

Fixes #41

Builder didn't want to compile the project as a flatpak project.

Fixes #24