SEATTLE (AP).- People are already joking it’s a good thing the Bill & Melinda Gates Foundation isn’t charging admission for its new visitor center, which showcases Bill Gates’ multibillion-dollar philanthropy, not his computers. But most people don’t know what to expect from the glass-clad public space in front of the new headquarters of the world’s largest charitable foundation. Those who decide to peek in the windows or stop inside will find thought-provoking and even fun exhibits that encourage visitors to focus on how they can make the world a better place. The center opens Saturday in Seattle, just steps from the Space Needle and Experience Music Project, two of the city’s biggest tourist attractions. While it’s not exactly a “museum of philanthropy,” it’s also not just a public