Annotations: Add layer Opacity paramater

This option was removed in the refactor of grease pencil and never was set again.

release/scripts/startup/bl_ui/properties_grease_pencil_common.py

@@ -478,6 +478,7 @@ class AnnotationDataPanel:
 
         tool_settings = context.tool_settings
         if gpd and gpl:
+            layout.prop(gpl, "opacity", text="Opacity", slider=True)
             layout.prop(gpl, "thickness")
         else:
             layout.prop(tool_settings, "annotation_thickness", text="Thickness")