tui: defer termcodes using a timer

With this implementation there is no "jank" during startup.

Using the main_loop in any fashion is janky. Using only the TUI loop
emits the termcodes too soon, or requires bad hacks like counting
tui_flush invocations (9 seems to work).

ref #7664
ref #7649
ref #7664
ref 27f9b1c7b0

src/nvim/ui.h

@@ -36,7 +36,6 @@ struct ui_t {
 #endif
   void (*event)(UI *ui, char *name, Array args, bool *args_consumed);
   void (*stop)(UI *ui);
-  void (*after_startup)(UI *ui);
 };
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS