Computed bounding box of compound nodes is wrong

[spoenemann]

With the new package node added to the class diagram example in #134, a weird behavior is revealed:
When the Bar class is moved out of its package, the size of the package is updated accordingly. But if the new position is left of or above the package, the position of the package is not updated, so the resizing has no effect (i.e. the class is rendered outside of the package).

This is caused by HiddenBoundsUpdater.getBoundsFromDOM, which applies the size of the bounding box obtained from the DOM, but not its position.