Duplicate news fixed in Beta 17

We fixed a problem that some users had when updating NI to a new version. They were getting all the news back again as unread. Now, you should stop getting duplicate news when you update NI!

Here is the complete list of changes:

- add: the installer can now support a custom.nix file for custom installations (for example, intranet);
- add: the installer support Silent mode (/S);
- mod: code optimization for the application's DLL;
- fix: news sometimes duplicate after an update of NI (truely fixed).