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We added a mechanism that moves the workspace when highlighting a
procedure definition block. However, we do not provide a mechanism to
go back to the original viewport position prior to the move. This
commit adds a WorkspaceMove event that is recorded when the workspace
is shifted and can be undo via the normal undo/redo stack to return
the user from where they came in the workspace.

Note this is not robust to changes in the viewport size between
operations.

Change-Id: I09f1af2f3d7db62af0f1487a90a56a66f99273bc