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/getchar.c

@@ -2379,6 +2379,11 @@ inchar (
   int retesc = FALSE;               /* return ESC with gotint */
   int script_char;
 
+  if (wait_time == -1L || wait_time > 100L) {
+    // flush output before waiting
+    ui_flush();
+  }
+
   /*
    * Don't reset these when at the hit-return prompt, otherwise an endless
    * recursive loop may result (write error in swapfile, hit-return, timeout