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.

test/functional/ui/screen.lua

@@ -166,7 +166,7 @@ function Screen.new(width, height)
     _cursor = {
       row = 1, col = 1
     },
-    _busy = true
+    _busy = false
   }, Screen)
   self:_handle_resize(width, height)
   return self