windowManager: Fix fullscreen windows in ws switching animations

To prevent a small gap between windows in the workspace switching
animation, we temporarily shift windows up or down by the height of the
panel. This obviously breaks the animation for fullscreen windows, those
will overlap with the ones on the other workspace since there is no
panel shown in that case.

Fix this by checking whether the old or new workspace includes a
fullscreen window and don't shift the windows if there is one.

Fixes #757

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