Add multichannel export to GUI

Closes #12

DiscordChatExporter.Gui/Behaviors/ChannelViewModelMultiSelectionListBoxBehavior.cs

@@ -0,0 +1,8 @@
+using DiscordChatExporter.Gui.ViewModels.Components;
+
+namespace DiscordChatExporter.Gui.Behaviors
+{
+    public class ChannelViewModelMultiSelectionListBoxBehavior : MultiSelectionListBoxBehavior<ChannelViewModel>
+    {
+    }
+}

DiscordChatExporter.Gui/Behaviors/MultiSelectionListBoxBehavior.cs

@@ -0,0 +1,92 @@
+using System.Collections;
+using System.Collections.Specialized;
+using System.Linq;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Interactivity;
+
+namespace DiscordChatExporter.Gui.Behaviors
+{
+    public class MultiSelectionListBoxBehavior<T> : Behavior<ListBox>
+    {
+        public static readonly DependencyProperty SelectedItemsProperty =
+            DependencyProperty.Register(nameof(SelectedItems), typeof(IList),
+                typeof(MultiSelectionListBoxBehavior<T>),
+                new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
+                    OnSelectedItemsChanged));
+
+        private static void OnSelectedItemsChanged(DependencyObject sender, DependencyPropertyChangedEventArgs args)
+        {
+            var behavior = (MultiSelectionListBoxBehavior<T>) sender;
+            if (behavior._modelHandled) return;
+
+            if (behavior.AssociatedObject == null)
+                return;
+
+            behavior._modelHandled = true;
+            behavior.SelectItems();
+            behavior._modelHandled = false;
+        }
+
+        private bool _viewHandled;
+        private bool _modelHandled;
+
+        public IList SelectedItems
+        {
+            get => (IList) GetValue(SelectedItemsProperty);
+            set => SetValue(SelectedItemsProperty, value);
+        }
+
+        // Propagate selected items from model to view
+        private void SelectItems()
+        {
+            _viewHandled = true;
+
+            AssociatedObject.SelectedItems.Clear();
+            if (SelectedItems != null)
+            {
+                foreach (var item in SelectedItems)
+                    AssociatedObject.SelectedItems.Add(item);
+            }
+
+            _viewHandled = false;
+        }
+
+        // Propagate selected items from view to model
+        private void OnListBoxSelectionChanged(object sender, SelectionChangedEventArgs args)
+        {
+            if (_viewHandled) return;
+            if (AssociatedObject.Items.SourceCollection == null) return;
+
+            SelectedItems = AssociatedObject.SelectedItems.Cast<T>().ToArray();
+        }
+
+        // Re-select items when the set of items changes
+        private void OnListBoxItemsChanged(object sender, NotifyCollectionChangedEventArgs args)
+        {
+            if (_viewHandled) return;
+            if (AssociatedObject.Items.SourceCollection == null) return;
+            SelectItems();
+        }
+
+        protected override void OnAttached()
+        {
+            base.OnAttached();
+
+            AssociatedObject.SelectionChanged += OnListBoxSelectionChanged;
+            ((INotifyCollectionChanged) AssociatedObject.Items).CollectionChanged += OnListBoxItemsChanged;
+        }
+
+        /// <inheritdoc />
+        protected override void OnDetaching()
+        {
+            base.OnDetaching();
+
+            if (AssociatedObject != null)
+            {
+                AssociatedObject.SelectionChanged -= OnListBoxSelectionChanged;
+                ((INotifyCollectionChanged) AssociatedObject.Items).CollectionChanged -= OnListBoxItemsChanged;
+            }
+        }
+    }
+}