Partial fix for T60982: Editable anim settings in linked action.

This is only partially working, because some bAnimListElem items do not
have any ID pointer set (for wome mysterious reason...), notably the
'group' ones.

Will re-assign to @aligorith for that, this code is rather complicated
and hard to follow (with all those macros ;) ).

source/blender/editors/animation/anim_channels_defines.c

@@ -4420,6 +4420,12 @@ static void draw_setting_widget(bAnimContext *ac, bAnimListElem *ale, const bAni
 			}
 		}
 	}
+
+	if ((ale->fcurve_owner_id != NULL && ID_IS_LINKED(ale->fcurve_owner_id)) ||
+	    (ale->id != NULL && ID_IS_LINKED(ale->id)))
+	{
+		UI_but_flag_enable(but, UI_BUT_DISABLED);
+	}
 }
 
 /* Draw UI widgets the given channel */