neon: Allow completely hiding linear progress indicator

packages/neon/neon/lib/src/widgets/linear_progress_indicator.dart

@@ -3,6 +3,7 @@ part of '../../neon.dart';
 class NeonLinearProgressIndicator extends StatelessWidget {
   const NeonLinearProgressIndicator({
     this.visible = true,
+    this.alwaysThere = true,
     this.margin = const EdgeInsets.symmetric(horizontal: 10),
     this.color,
     this.backgroundColor = Colors.transparent,
@@ -10,12 +11,15 @@ class NeonLinearProgressIndicator extends StatelessWidget {
   });
 
   final bool visible;
+  final bool alwaysThere;
   final EdgeInsets? margin;
   final Color? color;
   final Color? backgroundColor;
 
   @override
-  Widget build(final BuildContext context) => Container(
+  Widget build(final BuildContext context) => !alwaysThere && !visible
+      ? Container()
+      : Container(
           margin: margin,
           child: SizedBox(
             height: 3,