Add "Category display" view

- Select "parts" from the app drawer
- Allow traversal of sub-categories

lib/main.dart

@@ -1,3 +1,4 @@
+import 'package:InvenTree/widget/category_display.dart';
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
 // import 'package:preferences/preferences.dart';
@@ -166,6 +167,10 @@ class _MyHomePageState extends State<MyHomePage> {
     //Navigator.push(context, MaterialPageRoute(builder: (context) => InvenTreeSettingsWidget()));
   }
 
+  void _showParts() {
+    Navigator.push(context, MaterialPageRoute(builder: (context) => CategoryDisplayWidget(-1)));
+  }
+
   @override
   Widget build(BuildContext context) {
     // This method is rerun every time setState is called, for instance as done
@@ -197,6 +202,17 @@ class _MyHomePageState extends State<MyHomePage> {
                   title: new Text("InvenTree"),
               ),
               new Divider(),
+              new ListTile(
+                title: new Text("Scan"),
+              ),
+              new ListTile(
+                title: new Text("Parts"),
+                onTap: _showParts,
+              ),
+              new ListTile(
+                title: new Text("Stock"),
+              ),
+              new Divider(),
               new ListTile(
                 title: new Text("Settings"),
                 leading: new Icon(Icons.settings),