Added a new tool for resizing datasets: DataModifier

With DataModifier you can select a dataset, specify a resampling factor
and then export it. Currently only supports writing to mhd.
Add splitters between render widgets.

Now it's possible to resize the render widgets for a better view of the
datasets.
Remove the transform box code from the render widget.
Cleanup of strategy classes
Add simple user transformation tool.

Transformation only shows a transformation widget at the moment.