Bug 589316 - Add application/window icon to overlay windows

To better distinguish between vast fields of white of which many
windows are composed, add the application icon to the bottom
right of the window.

We fade them in to avoid an abrupt feel.  The icons are in the
workspaces group, not individual workspace groups to avoid
having to adjust them when we scale the workspaces.

Replace Workspace._lookupIndexAndClone with Workspace.lookupIndex,
and make the caller go from index to clone, or clone and index.