w32_common: ensure taskbar is hidden when fullscreen

Windows uses a heuristic to determine if a window should appear
fullscreen. If the active window's client area covers the whole screen,
the taskbar should move to the bottom of the Z-order, allowing the
window to show through.

Unfortunately, sometimes it doesn't work and the taskbar stays on top of
the "fullscreen" window. ITaskbarList2->MarkFullscreenWindow explicitly
tells the shell that a window wants to be fullscreen, so the taskbar is
always at the bottom of the Z-order while the marked window is active.

This might help with #999. Firefox also uses this interface to fix
fullscreen issues.