UI: add "compositor" layer to merge grids for TUI use in a correct way

Initially we will use this for the popupmenu, floating windows will
follow soon

NB: writedelay + compositor is weird, we need more flexible
redraw introspection.

src/nvim/os_unix.c

@@ -139,7 +139,7 @@ void mch_exit(int r)
   exiting = true;
 
   ui_flush();
-  ui_builtin_stop();
+  ui_call_stop();
   ml_close_all(true);           // remove all memfiles
 
   if (!event_teardown() && r == 0) {