Implementation of notifications for Windows OS.

Basically, you can now show notifications on Windows.
There is a limit on notification title and message lengths; if title and message don't fit in WinApi limitations, they get truncated in order to fit.
Also, when users click on the notification, they get switched to the Wesnoth game window.

The notifications should work for Windows XP and higher. If support of Windows 98 and higher is desired, it can be done with few changes.

Note that this pull request introduces two new files that should be compiled on Windows platform only, which means that project files should be changed accordingly. I hope that someone who knows project files' markup will volunteer and modify them.