Port Workspace::topClientOnDesktop() to VirtualDesktop

This patch has one behavioral change - raiseOrLowerClient() will not
work if the client is not on the current virtual desktop.

However, raiseOrLowerClient() can be called only in two cases:

* user triggers the raise or lower shortcut for the active client. Since
  the active client is on the current virtual desktop, it's not an issue

* an x11 window restacks itself. It makes no sense if an x11 window
  restacks itself while it's inactive or not on current virtual desktop.
  Also, the Opposite restack mode is rarely used, some window managers
  don't even bother implementing it. So, having such a constraint should
  not be a problem.

The main reason for not allowing raiseOrLowerClient() for windows that
are not on the current virtual desktop is that a window can be on
multiple virtual desktops. If a window is on A and B virtual desktops,
the only logical option is to toggle stacking position if the window is
on the current desktop. It's the only viable option as kwin does not
maintain per virtual desktop stacking order.