Fix #4382: the Eyedropper's Alt-click puts what it picked up on the object clicked when nothing is selected

With nothing selected, a click with the Eyedropper picks up an object's attributes and an Alt/Option-click on another object applies them, as the reference app's Eyedropper does. Alt-click only copied from a selected object, and with nothing selected it failed. The session now keeps a copy of what appearance.copyFrom picked up last, and a reversed copy with nothing selected applies it.

crates/engine/src/session/documents.rs

@@ -44,6 +44,7 @@ impl Session {
             menu: Default::default(),
             gradient_stop: None,
             active_appearance_item: None,
+            eyedropper_sample: None,
             last_solid: Color::WHITE,
             last_gradient: GradientPaint::new(Default::default()),
             recent_colors: vec![],