VS2019 Support - Set VisualStudioProductVersion to VS2019 if libcef_dll_wrapper.lib otherwise fallback to VS2017

Follow up to cefsharp#2924

Author: amaitland

CefSharp.props

@@ -5,7 +5,8 @@
     <VisualStudioProductVersion>2013</VisualStudioProductVersion>
     <VisualStudioProductVersion Condition="'$(VisualStudioVersion)'=='14.0'">2015</VisualStudioProductVersion>
     <VisualStudioProductVersion Condition="'$(VisualStudioVersion)'=='15.0'">2017</VisualStudioProductVersion>
-    <VisualStudioProductVersion Condition="'$(VisualStudioVersion)'=='16.0'">2019</VisualStudioProductVersion>
+    <VisualStudioProductVersion Condition="'$(VisualStudioVersion)'=='16.0'">2017</VisualStudioProductVersion>
+    <VisualStudioProductVersion Condition="'$(VisualStudioVersion)'=='16.0' AND Exists('$(SolutionDir)packages\$(CefSdkVer)\CEF\$(Platform)\$(Configuration)\VS2019\libcef_dll_wrapper.lib')">2019</VisualStudioProductVersion>
 
     <PlatformToolset>v120</PlatformToolset>
     <PlatformToolset Condition="'$(VisualStudioVersion)'=='14.0'">v140</PlatformToolset>