moving property to targets file, so that .csproj files can overwrite

packaging/Microsoft.NET.Sdk.WindowsDesktop/targets/Microsoft.NET.Sdk.WindowsDesktop.props

 <Project>
-  <PropertyGroup>
-    <DefaultXamlRuntime Condition="'$(DefaultXamlRuntime)'==''">Wpf</DefaultXamlRuntime>
-  </PropertyGroup>
   <ItemGroup Condition=" '$(_EnableWindowsDesktopGlobbing)' == 'true' ">
     <ApplicationDefinition Include="App.xaml"
                            Condition="'$(EnableDefaultApplicationDefinition)' != 'false' And Exists('$(MSBuildProjectDirectory)/App.xaml') And '$(MSBuildProjectExtension)' == '.csproj'">

packaging/Microsoft.NET.Sdk.WindowsDesktop/targets/Microsoft.NET.Sdk.WindowsDesktop.targets

@@ -4,6 +4,7 @@
     <_MicrosoftNetSdkWindowsDesktop>true</_MicrosoftNetSdkWindowsDesktop>
     <EnableDefaultPageItems Condition="'$(EnableDefaultPageItems)' == ''">true</EnableDefaultPageItems>
     <EnableDefaultApplicationDefinition Condition="'$(EnableDefaultApplicationDefinition)' == ''">true</EnableDefaultApplicationDefinition>
+    <DefaultXamlRuntime Condition="'$(DefaultXamlRuntime)'==''">Wpf</DefaultXamlRuntime>
   </PropertyGroup>
 
   <PropertyGroup>