[WindowsPlatform] Prevent multiple credential dialogs on Windows.

On Windows multiple dialogs asking for credentials were being shown together if multiple background threads requested credentials at the same time. On the Mac the dialogs are shown one at a time. Now Windows has the same behaviour as the Mac.

Tested this with a [simple addin](https://github.com/mrward/monodevelop-credential-provider-test) ([mpack](https://github.com/mrward/monodevelop-credential-provider-test/raw/master/MonoDevelop.CredentialProviderTest_0.1.mpack) provided) that requests credentials for the same url 5 times when the menu option **Credential Provider Test...** is selected from the **Project** menu.