Draft: feat: Support for message threads

Implements: #11058 (and #11271)

This MR adds support for threads like cheogram does.

A video of what it looks like:

gajim-identicons-2024-02-14_14.45.08

Check-list:

• Convince @lovetox that this is a good idea
• Make threads collapsable
• Convince @lovetox that identicons are a good idea
• Add a hint next to the message input box to specify that we are going to post in a given thread (…the identicon?)
• Change the identicon generation algorithm so that it matches Cheogram's (see below)
• Remove empty date rows in conversation view
• Figure out how to get the size of the message row icons and un-hardcode the 10×10px size

The identicons generated from the thread ID are different than those from Cheogram (implementation here)