ui: Refactor so that busy state won't be the default

Even though assuming nvim is busy most times is simpler, it has a problem: A lot
of unnecessary busy_start/busy_stop notifications are sent to the UI. That's
because in the majority of scenarios almost no time is spent between
`event_poll` calls.

This restores the normal behavior which is to call busy_start only when nvim is
going to perform some task that can take a significant amount of time. Also
improve the usage of buffering in the TUI when changing the cursor state.

src/nvim/misc1.c

@@ -2364,6 +2364,7 @@ int get_keystroke(void)
 
   mapped_ctrl_c = FALSE;        /* mappings are not used here */
   for (;; ) {
+    // flush output before waiting
     ui_flush();
     /* Leave some room for check_termcode() to insert a key code into (max
      * 5 chars plus NUL).  And fix_input_buffer() can triple the number of