This is useful if you want to view multiple locations in the same rom at the same time. This was previously possible by following a pointer into a new tab, but often the thing you want to see isn't available from an onscreen pointer, so it makes more sense to open up a new goto.
* Make sure the data in the tabs is aligned. This required changing the implementation to no longer depend on the SearchResultsViewPort, but instead the DiffTab owns the children viewports itself. This lets the DiffTab control the alignment between the parallel children.
* Make sure we can see differences from multiple changes in the tab at the same time.
