Support reordering of webxdc instances and updates

This can probably be done by creating a fake webxdc instance and adding it to the trash chat, and replacing it with the true instance later. We delete tombstones after 2 days, so updates will be deleted as well. The downside is that if ephemeral messages are enabled in the chat, such reordered updates may not be deleted on time. So another option is adding a fake hidden instance to the chat and setting an ephemeral timer for it to some smaller period so that it's updated when chat's ephemeral timer updates. Note that `ephemeral::start_chat_ephemeral_timers()` can only reduce the existing message ephemeral timestamp, so reordered updates will be deleted on time anyway if the true instance doesn't arrive.

This is a follow-up to discussion at https://github.com/chatmail/core/pull/7373#discussion_r2528888031_