Instead of only transforming the active workspace, create a
zooming effect when showing or hiding the overlay. This makes
the transitions simpler: the workspaces are now fixed to the
overlay actor group and will not slide over the Dash.
overlay.js: Add zoom animations, fade in/out Dash during those,
remove obsolete Dash clipping and stacking logic, add public
get[Scale|Position]() and getZoomedIn[Scale|Position]()
functions.
workspaces.js: Remove zoom animations, add fade animations for
the remove button, add helper functions for the overlay
zooming, keep the movement of windows linear to that of
their workspaces, remove the updatePosition() and
updateInOverlay() functions and fullSize variables that
were left from the old overlay design.
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