PDF improvements and more

- Reverted the previous commit, as nobody liked it :-(
- Added PDFView and PDFThumbnailView which work together to display, edit and print a PDFDocument.
- Display of built-in PDFs is now using the "single-page continuous" option and has a thumbnail view. As such, they can be printed without any code !
- Documented PDFView and PDFThumbnailView.
- Created a CanvasForNSView object to serve as a parent class for all NSView subclasses using a Canvas at the Xojo level.