Allow Sandbox applications to exit.

[EkardNT]

I noticed that when using the Sandbox its not possible to graceful tell iced to quit. Although I understand that Sandbox is a simplified alternative to Application, it seemed to me that Sandbox users may still want the ability to exit.

To make it possible for Sandbox programs to quit, this PR adds a fn should_exit(&self) -> bool method to the Sandbox trait. The Sandbox trait defines the default implementation of should_exit to return false, just like Application does, so Sandbox users who do not care about the ability to exit should notice no difference.

[hecrj]

Looks great! Thank you 🙇