Instead of labeling each method that needs to have visibility
we can label entire classes, this reduces the amount of noise and
potential for mistakes when writing classes.
Also made the TextAnnotation classes conform better to VTK-m coding
style. Specifically, changed the order of words in subclass names (e.g.
TextAnnotationBillboard instead of BillboardTextAnnotation) and broke
out each subclass into its own header/source files.