tinywl: stop using the word "view" to refer to toplevels

"View" has been cargo-culted from Weston. In Weston, a view is not even necessarily a toplevel -- it's just a way to draw an arbitrary somewhere (a surface may be painted at multiple locations simultaneously). The Weston concept has been misunderstood and then was carried over to rootston, Sway, and tinywl.

Let's just use the official Wayland wording instead.