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/os/shell.c

@@ -241,6 +241,7 @@ static int shell(const char *cmd,
   // invoke busy_start here so event_poll_until wont change the busy state for
   // the UI
   ui_busy_start();
+  ui_flush();
   status = job_wait(job, -1);
   ui_busy_stop();