screen: use dedicated message grid

add proper msg_set_pos event, delet win_scroll_over_*

make compositor click through unfocusable grids

add MsgArea attribute for the message/cmdline area, and add docs and tests

src/nvim/memline.c

@@ -3522,11 +3522,9 @@ static char *findswapname(buf_T *buf, char **dirp, char *old_fname,
             }
             xfree(name);
 
+
             // pretend screen didn't scroll, need redraw anyway
-            // TODO(bfredl): when doing the message grid refactor,
-            // simplify this special case.
-            msg_scrolled = 0;
-            redraw_all_later(NOT_VALID);
+            msg_reset_scroll();
           }
 
           if (choice > 0) {