Interactivity fix (#3268)

* Adds NuGet package "System.Windows.Interactivity.WPF" to solution.  Fixes build failure on a solo install of VS2017.

Author: shadowofsilicon

RetailCoder.VBE/packages.config

@@ -14,4 +14,5 @@
   <package id="Ninject.Extensions.NamedScope" version="3.2.0.0" targetFramework="net45" />
   <package id="NLog" version="4.0.1" targetFramework="net45" />
   <package id="NLog.Schema" version="4.0.1" targetFramework="net45" />
+  <package id="System.Windows.Interactivity.WPF" version="2.0.20525" targetFramework="net45" />
 </packages>