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 :)

deltachat-ffi/deltachat.h

@@ -7296,6 +7296,22 @@ void dc_event_unref(dc_event_t* event);
 /// `%1$s` will be replaced by the provider's domain.
 #define DC_STR_INVALID_UNENCRYPTED_MAIL 174
 
+/// "You reacted %1$s to '%2$s'"
+///
+/// `%1$s` will be replaced by the reaction, usually an emoji
+/// `%2$s` will be replaced by the summary of the message the reaction refers to
+///
+/// Used in summaries.
+#define DC_STR_YOU_REACTED 176
+
+/// "%1$s reacted %2$s to '%3$s'"
+///
+/// `%1$s` will be replaced by the name the contact who reacted
+/// `%2$s` will be replaced by the reaction, usually an emoji
+/// `%3$s` will be replaced by the summary of the message the reaction refers to
+///
+/// Used in summaries.
+#define DC_STR_REACTED_BY 177
 
 /**
  * @}