Release wheel listener when disposing workspace.

[BeksOmega]

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Description

We have a dummy wheel listener we add to the document to work around a bug in safari. Currently this listener is not getting disposed when the workspace is disposed.

To Fix

1. Create a class member variable to store the dummy listener.
2. Remove the listener inside the WorkspaceSvg's dispose method.

[ananta]

@BeksOmega I would love to work on this :)