Move XamlAccessLevel to System.Windows.Extensions.dll

Move XamlAccessLevel to System.Windows.Extensions.dll

WPF needs continued access to XamlAccessLevel type. System.Xaml.Permissions.XamlLoadPermission also needs to use XamlAccessLevel.

WPF is trying to remove its references to System.Security.Permissions. In order to complete this work, XamlAccessLevel is being moved to System.Windows.Extensions.dll (while leaving XamlLoadPermission in System.Permission.dll as-is). This will allow WindowsDesktop.App to bundle System.Windows.Extensions.dll (but not carry System.Security.Permissions.dll).

Commit migrated from https://github.com/dotnet/corefx/commit/758b1bfe0693801c3ac3e1b3b103a3b5e64be14a