Keyboard navigation in the desktop

In our shell we should be able to switch focus between taskbar components and the desktop by pressing the TAB key. This is not straightforward as the taskbar has lots and lots of child windows and only a few need to get the focus. Moreover the desktop and the taskbar live in different threads but we should be able to cycle the focus between them by pressing the TAB key. This needs the CDesktopBrowser to pass the focus to the taskbar when TAB is pressed, it needs the taskbar to switch the focus to its controls properly and when done give the focus back to the desktop. CBandSite from browseui needs to properly cycle focus to its child controls.