Fix a NullReferenceException in ExplorerBrowser by adjusting the order of static readonly fields.

[rzamponiAtIgt]

Repro steps:

1. Open .\source\Samples\ExplorerBrowser\CS\ExplorerBrowser.sln in VS
2. Set WPFExplorerBrowserDemo as the startup project
3. Hit F5

Call stack:

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Microsoft.WindowsAPICodePack.Shell
  StackTrace:
   at Microsoft.WindowsAPICodePack.Controls.WindowsPresentationFoundation.ExplorerBrowser..cctor()
 in D:\Playground\Windows-API-Code-Pack-1.1\source\WindowsAPICodePack\Shell\ExplorerBrowser\ExplorerBrowser.WPF.xaml.cs:line 17