Fix bug related to navigation windows. A single vtkRenderer cannot display a vtkImageActor and vtkActors simultaneously
Use separate renderers for image reslice and instruments (i.e., instrument line, screw, ruler, planned screws)
Now, a single instrument modification triggers a redraw of all instruments. This avoids keeping track of each instrument as a class member
Screws are now sorted by length
Screw names are now handled by ScrewProperties
ScrewProperties can generate screw IDs (a concatenation of length and diameter), e.g., for length = 4.5 and diameter = 5, id = "4.5x5". Todo: two screws with same properties will have same ID... add position in list
TODO: