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.
This fixes a regression where the geometry label isn't updating even
though the window geometry had changed because the geometry label's
location isn't changing.
An Overlay is similar to a transparent window, but has less overhead
and does not get rendered within the window stack. Basically, the area
that an Overlay occupies forces transparency rendering for any window
underneath, which allows us to render them flicker-free.
This also adds a new API that allows displaying the screen numbers,
e.g. while the user configures the screen layout in DisplaySettings
Because other things like drag&drop or the window-size label are not
yet converted to use this new mechanism, they will be drawn over the
screen-number currently.
