WindowServer: Add basic virtual desktop support

This creates a 2-dimensional array of WindowStack instances, one for
each virtual desktop. The main desktop 0,0 is the main desktop, which
is the desktop used for all stationary windows (e.g. taskbar, desktop).
When adding windows to a desktop, stationary windows are always added
to the main desktop.

When composing the desktop, there are usually two WindowStacks
involved. For stationary windows, the main desktop will be traversed,
and for everything else the current virtual desktop will be iterated.
Iteration is interweaved to preserve the correct order. During the
transition animation, two WindowStacks will be iterated at the same
time.

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