Barcode refactor (#363)

* Move barcode.dart

* Fix

* Refactoring barcode scanner code:

- Abstract the "controller" class (for future development)
- Break barcode scanning code out into multiple files
- Add CameraBarcodeController class (qr_code_scanner)

* Add await

* Make barcode scan delay configurable

* remove unused import

* Handle camera exceptions

* Improve sequencing for camera scanner

- Show loading overlay
- Prevent reload if view is no longer mounted

* Update docstring

* Update release notes

lib/settings/barcode_settings.dart

@@ -0,0 +1,114 @@
+import "package:flutter/material.dart";
+import "package:font_awesome_flutter/font_awesome_flutter.dart";
+import "package:inventree/l10.dart";
+import "package:inventree/preferences.dart";
+
+
+class InvenTreeBarcodeSettingsWidget extends StatefulWidget {
+  @override
+  _InvenTreeBarcodeSettingsState createState() => _InvenTreeBarcodeSettingsState();
+}
+
+
+class _InvenTreeBarcodeSettingsState extends State<InvenTreeBarcodeSettingsWidget> {
+
+ _InvenTreeBarcodeSettingsState();
+
+ int barcodeScanDelay = 500;
+
+ final TextEditingController _barcodeScanDelayController = TextEditingController();
+
+ @override
+ void initState() {
+  super.initState();
+  loadSettings();
+ }
+
+  Future<void> loadSettings() async {
+    barcodeScanDelay = await InvenTreeSettingsManager().getValue(INV_BARCODE_SCAN_DELAY, 500) as int;
+
+    if (mounted) {
+      setState(() {
+      });
+    }
+  }
+
+  // Callback function to edit the barcode scan delay value
+  // TODO: Next time any new settings are added, refactor this into a generic function
+  Future<void> _editBarcodeScanDelay(BuildContext context) async {
+
+    _barcodeScanDelayController.text = barcodeScanDelay.toString();
+
+    return showDialog(
+      context: context,
+      builder: (context) {
+        return AlertDialog(
+          title: Text(L10().barcodeScanDelay),
+          content: TextField(
+            onChanged: (value) {},
+            controller: _barcodeScanDelayController,
+            keyboardType: TextInputType.number,
+            decoration: InputDecoration(
+              hintText: L10().barcodeScanDelayDetail,
+            ),
+          ),
+          actions: <Widget>[
+            MaterialButton(
+              color: Colors.red,
+              textColor: Colors.white,
+              child: Text(L10().cancel),
+              onPressed: () {
+                setState(() {
+                  Navigator.pop(context);
+                });
+              },
+            ),
+            MaterialButton(
+              color: Colors.green,
+              textColor: Colors.white,
+              child: Text(L10().ok),
+              onPressed: () async {
+                int delay = int.tryParse(_barcodeScanDelayController.text) ?? barcodeScanDelay;
+
+                // Apply limits
+                if (delay < 100) delay = 100;
+                if (delay > 2500) delay = 2500;
+
+                InvenTreeSettingsManager().setValue(INV_BARCODE_SCAN_DELAY, delay);
+                setState(() {
+                  barcodeScanDelay = delay;
+                  Navigator.pop(context);
+                });
+              },
+            ),
+          ],
+        );
+      }
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: Text(L10().barcodes)),
+      body: Container(
+        child: ListView(
+          children: [
+            ListTile(
+              title: Text(L10().barcodeScanDelay),
+              subtitle: Text(L10().barcodeScanDelayDetail),
+              leading: FaIcon(FontAwesomeIcons.stopwatch),
+              trailing: GestureDetector(
+                child: Text("${barcodeScanDelay} ms"),
+                onTap: () {
+                  _editBarcodeScanDelay(context);
+                },
+              ),
+            )
+          ],
+        )
+      )
+    );
+  }
+
+}