Order extra lines (#632)

* Define classes for extra line item

* Display PO extra line items

- Also, some refactoring

* Support extra line items for sales order

* linting fixes

* Update release notes

lib/widget/order/extra_line_detail.dart

@@ -0,0 +1,112 @@
+import "package:flutter/material.dart";
+import "package:flutter_tabler_icons/flutter_tabler_icons.dart";
+import "package:inventree/helpers.dart";
+
+import "package:inventree/l10.dart";
+import "package:inventree/widget/refreshable_state.dart";
+import "package:inventree/widget/snacks.dart";
+
+import "package:inventree/inventree/orders.dart";
+
+
+class ExtraLineDetailWidget extends StatefulWidget {
+  const ExtraLineDetailWidget(this.item, {Key? key}) : super(key: key);
+
+  final InvenTreeExtraLineItem item;
+
+  @override
+  _ExtraLineDetailWidgetState createState() => _ExtraLineDetailWidgetState();
+}
+
+class _ExtraLineDetailWidgetState extends RefreshableState<ExtraLineDetailWidget> {
+
+  _ExtraLineDetailWidgetState();
+
+  @override
+  String getAppBarTitle() => L10().extraLineItem;
+
+  @override
+  List<Widget> appBarActions(BuildContext context) {
+    List<Widget> actions = [];
+
+    if (widget.item.canEdit) {
+      actions.add(
+        IconButton(
+          icon: Icon(TablerIcons.edit),
+          onPressed: () {
+            _editLineItem(context);
+          }
+        )
+      );
+    }
+
+    return actions;
+  }
+
+  // Function to request data for this page
+  @override
+  Future<void> request(BuildContext context) async {
+    await widget.item.reload();
+  }
+
+  // Callback to edit this line item
+  Future<void> _editLineItem(BuildContext context) async {
+    var fields = widget.item.formFields();
+
+    widget.item.editForm(
+        context,
+        L10().editLineItem,
+        fields: fields,
+        onSuccess: (data) async {
+          refresh(context);
+          showSnackIcon(L10().lineItemUpdated, success: true);
+        }
+    );
+  }
+
+  @override
+  List<Widget> getTiles(BuildContext context) {
+    List<Widget> tiles = [];
+
+    tiles.add(
+        ListTile(
+          title: Text(L10().reference),
+          trailing: Text(widget.item.reference),
+        )
+    );
+
+    tiles.add(
+        ListTile(
+          title: Text(L10().description),
+          trailing: Text(widget.item.description),
+        )
+    );
+
+    tiles.add(
+      ListTile(
+        title: Text(L10().quantity),
+        trailing: Text(widget.item.quantity.toString()),
+      )
+    );
+
+    tiles.add(
+      ListTile(
+        title: Text(L10().unitPrice),
+        trailing: Text(
+          renderCurrency(widget.item.price, widget.item.priceCurrency)
+        )
+      )
+    );
+
+    if (widget.item.notes.isNotEmpty) {
+      tiles.add(
+        ListTile(
+          title: Text(L10().notes),
+          subtitle: Text(widget.item.notes),
+        )
+      );
+    }
+
+    return tiles;
+  }
+}