Scanner wedge mode (#437)

* Add code_scan_listener package

* Implement wedge controller widget

* Update barcode settings widget

- Allow user to choose which barcode scanner to use

* Fix typo

* Select barcode scanner widget based on user preference

* Fix rendering issues for wedge controller

* Update release notes

* Add unit test for wedge scanner widget

- Required some tweaks to other code

* Use better library

- https://github.com/fuadreza/flutter_barcode_listener
- Fork of https://github.com/shaxxx/flutter_barcode_listener
- Properly handles key "case" issues (shift, essentially)
- Verified that it works correctly for multiple character types

* Local copy of code, rather than relying on package which is not available on pub.dev

* Fix unit test

lib/widget/progress.dart

@@ -14,7 +14,12 @@ Widget progressIndicator() {
 }
 
 
-void showLoadingOverlay(BuildContext context) {
+void showLoadingOverlay(BuildContext? context) {
+
+  if (context == null) {
+    return;
+  }
+
   Loader.show(
     context,
     themeData: Theme.of(context).copyWith(colorScheme: ColorScheme.fromSwatch())