explorer: Use a valid taskbar position when the taskbar is hidden.
Some applications use FindWindowA() with class Shell_TrayWnd to find the taskbar window on Windows.
Then GetWindowRect() is called to get the taskbar window rectangle. Finally, the taskbar window
rectangle is subtracted from the primary screen rectangle to calculate the work area. Without a
valid taskbar window position, these applications end up getting an incorrect work area and going
down the wrong path. So use the same position and size as the host system panel for explorer taskbar
when it's hidden.