Basic accessibility support for Slint UIs

There is still work to do to make Slint's accessibility support great, but this is an important first step: It validates the ideas we had to make Slint UIs accessible and puts a lot of infrastructure into place.

So Widgets can now have `accessible-*` properties that the compiler knows how to handle. Those properties are available at runtime and one of our backends does work with them and forwards them to the OS-specific accessibility backend for actual interaction with screen readers, braille-terminals or whatever is needed. We decided to use the Qt backend in this phase: That covers the major desktop platforms with one implementation. We can add different/more backends going forward, the infrastructure for that is in place now.