Simplify wayland code

[MaxVerevkin]

This mainly addresses this comment:

richclip/src/clipboard/wayland.rs

Lines 233 to 236 in e5f8c1a

	// This looks strange, but it is working. It seems offer.receive is a request but nont a
	// blocking call, which needs an extra loop to finish. Maybe a callback needs to be set
	// to wait until it is processed, but I have no idea how to do that.
	// conn.set_callback_for() doesn't work for the offer here.

calling a request function simply puts a request into the queue and does not perform any IO. This change moves the actual IO out of event callback into the paste function.

Also switch to using std::io::copy instead of a manual copy impl.

[beeender]

Thanks a lot for the nice refactor! And thanks for the wayrs!

Don't worry about the comment, I will fix it later.