message: Add search box for online datasets
diff: HeaderTemplate="{StaticResource TabItemHeaderTemplate}">
  <Grid Margin="0,10,0,0">
  <Grid.RowDefinitions>
  + <RowDefinition Height="Auto" />
  <RowDefinition Height="*" />
  <RowDefinition Height="Auto" />
  </Grid.RowDefinitions>
  + <TextBox
  + Margin="0,8"
  + Text="{Binding SearchText, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
  + mah:TextBoxHelper.ClearTextButton="True"
  + mah:TextBoxHelper.UseFloatingWatermark="True"
  + mah:TextBoxHelper.Watermark="Search"
  + >
  + <TextBox.Style>
  + <Style BasedOn="{StaticResource MahApps.Styles.TextBox.Search}" TargetType="{x:Type TextBox}">
  + <Style.Triggers>
  + <Trigger Property="mah:TextBoxHelper.HasText" Value="True">
  + <Setter Property="mah:TextBoxHelper.ButtonContent" Value="r" />
  + <Setter Property="mah:TextBoxHelper.ButtonContentTemplate" Value="{x:Null}" />
  + </Trigger>
  + </Style.Triggers>
  + </Style>
  + </TextBox.Style>
  + </TextBox>
  +
  <DataGrid
  x:Name="OnlineDatasetsGrid"
  + Grid.Row="1"
  AutoGenerateColumns="False"
  BeginningEdit="DatagridEditingHandler"
  CanUserAddRows="False"
  ColumnHeaderStyle="{StaticResource StandardColumnHeader}"
  EnableRowVirtualization="False"
  - ItemsSource="{Binding OnlineDatasets}"
  + ItemsSource="{Binding VisibleOnlineDatasets}"
  SelectionMode="Single">
  <DataGrid.Columns>
  <DataGridTextColumn
  <TextBlock
  x:Name="EmptyOnlineDatasetsListMessage"
  + Grid.Row="1"
  HorizontalAlignment="Center"
  VerticalAlignment="Center"
  Style="{StaticResource ListItemTextBlockStyle}"
  <Button
  x:Name="ConnectButton"
  - Grid.Row="1"
  + Grid.Row="2"
  Width="100"
  Margin="0,10,0,0"
  HorizontalAlignment="Right"
  