chore: minor ui components improvements (#140)

* improve ui

* .

* .

crates/ui/src/checkbox.rs

@@ -54,13 +54,13 @@ impl Disableable for Checkbox {
 }
 
 impl Selectable for Checkbox {
-    fn element_id(&self) -> &ElementId {
-        &self.id
-    }
-
     fn selected(self, selected: bool) -> Self {
         self.checked(selected)
     }
+
+    fn is_selected(&self) -> bool {
+        self.checked
+    }
 }
 
 impl RenderOnce for Checkbox {