Initial work on last-read-position feature

Print dashes on the position we last left off in a chat window.
So far the number of dashes is hardcoded, and the feature only works in
chat windows.

Regards https://github.com/profanity-im/profanity/issues/1238

src/ui/chatwin.c

@@ -286,6 +286,7 @@ chatwin_incoming_msg(ProfChatWin *chatwin, ProfMessage *message, gboolean win_cr
             }
         }
 
+        win_insert_last_read_position_marker((ProfWin*)chatwin, chatwin->barejid);
         win_print_incoming(window, display_name, message);
     }