Add the first iteration of a Folder Listing screen

This is just me experimenting with how the folder listing could look
like, and the different libraries I could use - For now I'm using the
https://github.com/ajilo297/flutter_tree_view library which I've forked
into this file, as it requires many many modifications to get it to how
I want it to work. Plus the library is really simple.

lib/screens/folder_listing.dart

@@ -0,0 +1,181 @@
+import 'package:flutter/material.dart';
+
+import 'package:gitjournal/widgets/app_bar_menu_button.dart';
+import 'package:gitjournal/widgets/app_drawer.dart';
+
+typedef void ParentSelectChanged(bool isSelected);
+
+/// # Tree View
+///
+/// Creates a tree view widget. The widget is a List View with a [List] of
+/// [Parent] widgets. The [TreeView] is nested inside a [Scrollbar] if the
+/// [TreeView.hasScrollBar] property is true.
+class TreeView extends StatelessWidget {
+  final List<Parent> parentList;
+  final bool hasScrollBar;
+
+  TreeView({
+    this.parentList = const <Parent>[],
+    this.hasScrollBar = false,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return hasScrollBar ? Scrollbar(child: _getTreeList()) : _getTreeList();
+  }
+
+  Widget _getTreeList() {
+    return ListView.builder(
+      itemBuilder: (context, index) {
+        return parentList[index];
+      },
+      itemCount: parentList.length,
+    );
+  }
+}
+
+/// # Parent widget
+///
+/// The [Parent] widget holds the [Parent.parent] widget and
+/// [Parent.childList] which is a [List] of child widgets.
+///
+/// The [Parent] widget is wrapped around a [Column]. The [Parent.childList]
+/// is collapsed by default. When clicked the child widget is expanded.
+class Parent extends StatefulWidget {
+  final Widget parent;
+  final ChildList childList;
+  final MainAxisSize mainAxisSize;
+  final CrossAxisAlignment crossAxisAlignment;
+  final MainAxisAlignment mainAxisAlignment;
+  final ParentSelectChanged callback;
+  final Key key;
+
+  Parent({
+    @required this.parent,
+    @required this.childList,
+    this.mainAxisAlignment = MainAxisAlignment.center,
+    this.crossAxisAlignment = CrossAxisAlignment.start,
+    this.mainAxisSize = MainAxisSize.min,
+    this.callback,
+    this.key,
+  });
+
+  @override
+  ParentState createState() => ParentState();
+}
+
+class ParentState extends State<Parent> {
+  bool _isSelected = false;
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      mainAxisSize: widget.mainAxisSize,
+      crossAxisAlignment: widget.crossAxisAlignment,
+      mainAxisAlignment: widget.mainAxisAlignment,
+      children: <Widget>[
+        GestureDetector(
+          child: widget.parent,
+          onTap: expand,
+        ),
+        _getChild(),
+      ],
+    );
+  }
+
+  void expand() {
+    if (widget.callback != null) widget.callback(_isSelected);
+    setState(() {
+      _isSelected = _toggleBool(_isSelected);
+    });
+  }
+
+  bool _toggleBool(bool b) {
+    return b ? false : true;
+  }
+
+  Widget _getChild() {
+    return _isSelected ? widget.childList : Container();
+  }
+}
+
+/// # ChildList widget
+///
+/// The [ChildList] widget holds a [List] of widget which will be displayed as
+/// children of the [Parent] widget
+class ChildList extends StatelessWidget {
+  final List<Widget> children;
+  final MainAxisAlignment mainAxisAlignment;
+  final CrossAxisAlignment crossAxisAlignment;
+  final MainAxisSize mainAxisSize;
+
+  ChildList({
+    this.children = const <Widget>[],
+    this.mainAxisSize = MainAxisSize.min,
+    this.crossAxisAlignment = CrossAxisAlignment.start,
+    this.mainAxisAlignment = MainAxisAlignment.center,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      mainAxisAlignment: mainAxisAlignment,
+      crossAxisAlignment: crossAxisAlignment,
+      mainAxisSize: mainAxisSize,
+      children: children,
+    );
+  }
+}
+
+class FolderListingScreen extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    var treeView = TreeView(
+      parentList: [
+        Parent(
+          parent: _buildFolderTile("Desktop"),
+          childList: ChildList(
+            children: <Widget>[
+              Parent(
+                parent: _buildFolderTile('documents'),
+                childList: ChildList(
+                  children: <Widget>[
+                    _buildFolderTile('Resume.docx'),
+                    _buildFolderTile('Billing-Info.docx'),
+                  ],
+                ),
+              ),
+              _buildFolderTile('MeetingReport.xls'),
+              _buildFolderTile('MeetingReport.pdf'),
+              _buildFolderTile('Demo.zip'),
+            ],
+          ),
+        ),
+      ],
+    );
+
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text('Folders'),
+        leading: GJAppBarMenuButton(),
+      ),
+      body: treeView,
+      drawer: AppDrawer(),
+    );
+  }
+
+  Widget _buildFolderTile(String name) {
+    // FIXME: The trailing icon should change based on if it is expanded or not
+
+    return Card(
+      child: ListTile(
+        leading: Icon(Icons.folder),
+        title: Text(name),
+        trailing: IconButton(
+          icon: Icon(Icons.navigate_next),
+          onPressed: () {},
+        ),
+      ),
+    );
+  }
+}