Allow "Reply" to create sub-threads

Desktop: A menu at the Right of the message allows, instead of just deleting the message,

• Delete message if allowed for this chat in its prefs.
• Edit message if allowed for this chat in its prefs.
• Reply to message.
• [Hide thread]

The latter creates a new indented message input field under the original message.
When the new message is inserted, it is indented a bit to the right
When the new thread is first added, a "Reply to thread" button is added to do the same thing more easily

Also allow a Hide Thread button somewhere. Grey Triangle at left?

RDF model: a link between the two messages, stored in the chat file, or in both chat files if they are in different dated chat files.
Ontology to be decided. SIOC? Schema.org?

Model: How Telegram does this. Also look at Reddit and hacker news...

Creating new threads should be an option the creator of the chat can tun off, but default on.

You should be able to open the first message of a thread in the browser and see just the thread below it.