Add support for taskbar features under Windows Vista/7+, and OS X

by Chaobin Zhang for wxWidgets

Windows Vista/7+ has added support for several new features to the taskbar which are all accessible via ITaskbarList3 interface, we can implement wrapper with this interface to help developers get rid of COM while implementing taskbar functionality, so that they can write more pure wxWidgets code. Notice that we can provide some of ITaskbarList3 functionality under OS X and Ubuntu Unity as well.