image-copy: Use abstraction that's now provided by Smithay

This doesn't change much, since the Smithay implementation is based on
the `cosmic-comp` version, but made more generic. We provide our own
implementation for our workspace capture protocol, but otherwise Smithay
handles the boilerplate now.

This should not cause any change in behavior.

src/wayland/protocols/mod.rs

@@ -7,7 +7,6 @@ pub mod image_capture_source;
 pub mod output_configuration;
 pub mod output_power;
 pub mod overlap_notify;
-pub mod screencopy;
 pub mod toplevel_info;
 pub mod toplevel_management;
 pub mod workspace;