feat: show reactions in summaries (#5387)

shows the last reaction in chatlist's summaries if there is no
newer message.

the reason to show reactions in the summary, is to make them a _little_
more visible when one is not in the chat. esp. in not-so-chatty or in
one-to-ones chats this becomes handy: imaging a question and someone
"answers" with "thumbs up" ... 

otoh, reactions are still tuned down on purpose: no notifications, chats
are opend as usual, the chatlist is not sorted by reactions and also the
date in the summary refer to the last message - i thought quite a bit
about that, this seems to be good compromise and will raise the fewest
questions. it is somehow clear to the users that reactions are not the
same as a real message. also, it is comparable easy to implement - no
UI changes required :)

all that is very close to what whatsapp is doing (figured that out by
quite some testing ... to cite @adbenitez: if in doubt, we can blame
whatsapp :)

technically, i first wanted to go for the "big solution" and add two
more columns, chat_id and timestamp, however, it seemed a bit bloated if
we really only need the last one. therefore, i just added the last
reaction information to the chat's param, which seems more performant
but also easier to code :)

src/param.rs

@@ -64,6 +64,15 @@ pub enum Param {
     /// For Messages: the message is a reaction.
     Reaction = b'x',
 
+    /// For Chats: the timestamp of the last reaction.
+    LastReactionTimestamp = b'y',
+
+    /// For Chats: Message ID of the last reaction.
+    LastReactionMsgId = b'Y',
+
+    /// For Chats: Contact ID of the last reaction.
+    LastReactionContactId = b'1',
+
     /// For Messages: a message with "Auto-Submitted: auto-generated" header ("bot").
     Bot = b'b',