No need to poll every 500ms if state is not dirty

[lutzky]

When handling events, we poll every 500ms:

zsnapfree/src/app.rs

Lines 165 to 177 in 60e6c61

	fn handle_events(&mut self) -> Result<()> {
	if poll(Duration::from_millis(500))? {
	match event::read()? {
	Event::Key(key_event) if key_event.kind == KeyEventKind::Press => {
	self.handle_key_event(key_event);
	}
	_ => {}
	};
	} else {
	self.recalculate_result();
	}
	Ok(())
	}

However, if self.dirty is false, we can poll indefinitely.