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/main.c

@@ -154,6 +154,7 @@ void event_init(void)
   remote_ui_init();
   api_vim_init();
   terminal_init();
+  ui_init();
 }
 
 /// @returns false if main_loop could not be closed gracefully