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/window.h

@@ -90,4 +90,7 @@ void win_page_down(ProfWin *window);
 void win_sub_page_down(ProfWin *window);
 void win_sub_page_up(ProfWin *window);
 
+void win_insert_last_read_position_marker(ProfWin *window, char* id);
+void win_remove_entry_message(ProfWin *window, const char *const id);
+
 #endif