Joints: joint.reach (inverse kinematics), and a jointed part follows a drag (#1540)

Thanks @SychPL.

crates/ui-egui/src/dialogs_assembly.rs

@@ -343,7 +343,7 @@ fn pt(v: Vec3) -> Value {
 }
 
 /// The occurrence placing a body (0: the root design), when the body is its primary instance.
-fn occurrence_of_body(s: &Session, body: &str) -> Option<u64> {
+pub(crate) fn occurrence_of_body(s: &Session, body: &str) -> Option<u64> {
     let st = s.world_state();
     let b = st.body(body)?;
     let comp = s.doc.model_body_component(b);