make thumbnail generating multithread

fix the bug with open video
add class distribution chart

Azaion.Annotator/DatasetExplorer.xaml

@@ -4,11 +4,11 @@
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:vwp="clr-namespace:WpfToolkit.Controls;assembly=VirtualizingWrapPanel"
-        xmlns:local="clr-namespace:Azaion.Annotator"
         xmlns:dto="clr-namespace:Azaion.Annotator.DTO"
         xmlns:controls="clr-namespace:Azaion.Annotator.Controls"
+        xmlns:ScottPlot="clr-namespace:ScottPlot.WPF;assembly=ScottPlot.WPF"
         mc:Ignorable="d"
-        Title="Браузер анотацій" Height="900" Width="1200">
+        Title="Переглядач анотацій" Height="900" Width="1200">
 
     <Window.Resources>
         <DataTemplate x:Key="ThumbnailTemplate" DataType="{x:Type dto:ThumbnailDto}">
@@ -58,7 +58,7 @@
             HorizontalAlignment="Stretch"
             VerticalAlignment="Stretch"
             Background="Black">
-            <TabItem Header="Браузер">
+            <TabItem Header="Анотації">
                 <vwp:GridView
                     Name="ThumbnailsView"
                     HorizontalAlignment="Stretch"
@@ -70,12 +70,15 @@
                 >
                 </vwp:GridView>
             </TabItem>
-            <TabItem Header="Перегляд">
+            <TabItem Header="Редактор">
                 <controls:CanvasEditor x:Name="ExplorerEditor"
                        VerticalAlignment="Stretch"
                        HorizontalAlignment="Stretch" >
                 </controls:CanvasEditor>
             </TabItem>
+            <TabItem Header="Розподіл класів">
+                <ScottPlot:WpfPlot x:Name="ClassDistribution" />
+            </TabItem>
         </TabControl>
         <StatusBar
             Grid.Row="1"