[Editor] Indicate if the highlight-thickness slider is disabled

The fact that the highlight-thickness can only be changed in "free" mode isn't really obvious visually in the toolbar, so attempt to provide at least some indication of the `disabled`-state by "dimming" the slider.

web/annotation_editor_layer_builder.css

@@ -1188,6 +1188,10 @@
         --example-color: CanvasText;
       }
 
+      :is(& > .editorParamsSlider[disabled]) {
+        opacity: 0.4;
+      }
+
       &::before {
         content: "";
         width: 8px;