fix: Sort system messages to the bottom of the chat

Fix #7435

For most messages, `calc_sort_timestamp()` makes sure that they are at the correct place; esp. that they are not above system messages or other noticed/seen messages.

Most callers of `add_info_msg()`, however, didn't call `calc_sort_timestamp()`, and just used `time()` or `smeared_time()` to get the sort timestamp. Because of this, system messages could sometimes wrongly be sorted above other messages.

This PR fixes this by making the sort timestamp optional in `add_info_msg*()`. If the sort timestamp isn't passed, then the message is sorted to the bottom of the chat. `sent_rcvd_timestamp` is not optional anymore, because we need _some_ timestamp that can be shown to the user (most callers just pass `time()` there).

src/webxdc.rs

@@ -379,8 +379,8 @@ impl Context {
                     instance.chat_id,
                     info.as_str(),
                     SystemMessage::WebxdcInfoMessage,
+                    Some(timestamp),
                     timestamp,
-                    None,
                     Some(&instance),
                     Some(from_id),
                     None,