Both MITK and Slicer are VTK-based open-source software with restriction-free license, so you can use them or copy-paste any code from them as you see fit.
If you only need to make measurements in a single view then you might do away with stock VTK widgets, otherwise it probably makes more sense to use/customize/extend MITK or Slicer. Both these application frameworks are developed to serve as a basis for custom applications.