Fix drag and drop area in main window (stakira#1564)

Co-authored-by: Maiko <[email protected]>

Author: 2 people

OpenUtau/Views/MainWindow.axaml

@@ -9,15 +9,15 @@
         Icon="/Assets/open-utau.ico"
         Title="{Binding Title}" MinWidth="300" MinHeight="200"
         KeyDown="OnKeyDown" PointerPressed="OnPointerPressed" Closing="WindowClosing"
-        Focusable="True"
+        Focusable="True" DragDrop.AllowDrop="True"
         TransparencyLevelHint="None">
   <Window.Styles>
     <Style Selector="Button,ToggleButton">
       <Setter Property="Focusable" Value="False"/>
     </Style>
   </Window.Styles>
   <Grid>
-    <Grid Name="MainGrid" DragDrop.AllowDrop="True" Margin="{Binding $parent[1].WindowDecorationMargin}">
+    <Grid Name="MainGrid" Margin="{Binding $parent[1].WindowDecorationMargin}">
       <Menu Grid.Row="0" Grid.Column="0" VerticalAlignment="Top" HorizontalAlignment="Left" Height="24"
                 Opened="OnMainMenuOpened" Closed="OnMainMenuClosed" PointerExited="OnMainMenuPointerLeave"
                 ZIndex="1">