Fix slice view annotations
Created by: jcfr
In addition to applying various style fixes, removing dead code and re-factoring the code to improve its readability, this PR (more specifically c89bfdc) fixes SlicerProstate/SliceTracker#33 to avoid blank rectangle from showing up in SliceViewAnnotations
Cc: @fedorov @cpinter @mehrtash @che85 @pieper @Slicer/slicer-core-team