Implement SetColor for Handle<Region> instead

This is required to extract appearance into a separate layer, as
associating attributes to objects can only work, if we have a unique
`Handle` to that object.

crates/fj-core/src/operations/presentation.rs

@@ -2,16 +2,19 @@
 
 use fj_interop::Color;
 
-use crate::objects::Region;
+use crate::{
+ objects::{IsObject, Region},
+ storage::Handle,
+};
 
 /// Set the color of an object
-pub trait SetColor {
+pub trait SetColor: IsObject {
  /// Set the color of the object
- fn set_color(&self, color: impl Into<Color>) -> Self;
+ fn set_color(&self, color: impl Into<Color>) -> Self::BareObject;
 }
 
-impl SetColor for Region {
- fn set_color(&self, color: impl Into<Color>) -> Self {
+impl SetColor for Handle<Region> {
+ fn set_color(&self, color: impl Into<Color>) -> Self::BareObject {
  Region::new(
  self.exterior().clone(),
  self.interiors().into_iter().cloned(),