Refactor.
Disable opacity slider when MIP visualization is used.
Improve stretching of controls on resize.
Fix alignment of labels.
Hide margins of the image plane widgets.
Refactor clipping box.

Not optimal yet, but good enough for now.
Only enable the opacity sliders when simple visualization type is used.
Adjust the DefaultPixelValue to be the minimum scalar value of the data.
Refactor test case name.
Merge branch 'master' of https://github.com/berendkleinhaneveld/Registrationshop