Fix #901: Shift-click a target circle in the Layers panel to add or remove it

Clicking an object's target circle in the Layers panel always replaced
the selection, so several objects couldn't be picked there. Shift-click
now adds the object (a layer: its art) to the selection, or takes it out
when it is selected already, as in Illustrator; nothing stays targeted
then. layer.target takes add: true for it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/selection.rs

@@ -128,6 +128,20 @@ impl Selection {
         }
         self.target = Some(id);
     }
+    /// Shift-clicking `id`'s target circle: it (a layer: its art) joins the selection, or leaves it
+    /// when it is all selected already. Several objects are then selected, none targeted.
+    pub fn toggle_target(&mut self, doc: &Document, id: NodeId) {
+        let ids = match doc.node(id) {
+            Some(n) if n.is_layer() => n.layer_art(true),
+            _ => vec![id],
+        };
+        if ids.iter().all(|i| self.contains(*i)) {
+            ids.iter().for_each(|i| self.remove(*i));
+        } else {
+            ids.into_iter().for_each(|i| self.add(i));
+        }
+        self.target = None;
+    }
     /// What appearance and transparency edits act on: the targeted object, group or layer, else
     /// the selected objects.
     pub fn subjects(&self) -> &[NodeId] {