Move GraphicSlot out of components module (#3444)

This PR moves the `GraphicSlot` definition out of the `components`
module, as it doesn't contain a component.

Release Notes:

- N/A

crates/ui2/src/ui2.rs

@@ -18,6 +18,7 @@ mod disableable;
 mod fixed;
 pub mod prelude;
 mod selectable;
+mod slot;
 mod styled_ext;
 mod styles;
 mod toggleable;
@@ -29,6 +30,7 @@ pub use disableable::*;
 pub use fixed::*;
 pub use prelude::*;
 pub use selectable::*;
+pub use slot::*;
 pub use styled_ext::*;
 pub use styles::*;
 pub use toggleable::*;