explorer: switching desktops doesn't always hide taskbar buttons from other desktops

Tested with 0.3.0 RC1 QEMU package.

Steps to reproduce:

Start ReactOS
Click Explorer quick-launch icon
Switch to desktop 2 with button on taskbar - note that "Reactos Explorer" taskbar button is still visible
Click "Reactos Explorer" taskbar button - initiates a shutdown

Expected behavior:
Button shouldn't be shown after switching to desktop 2