Set UseWPF build property to true for the toolkit projects.

This ensures that the XAML resources are included in the assembly.

Author: reduckted

src/toolkit/Directory.Build.props

@@ -10,6 +10,9 @@
     <RootNamespace>Community.VisualStudio.Toolkit</RootNamespace>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <DebugType>portable</DebugType>
+
+    <!-- Set UseWPF to true so that the XAML resources are compiled and included in the assembly. -->
+    <UseWPF>true</UseWPF>
   </PropertyGroup>
 
   <ItemGroup>