Refactor mechanism for saving files.
Move all print_ methods to their backends.
Manage default format-to-backend mappings in backend_bases.
print_figure() can now determine the backend for saving figures.
Remove print_ code duplication, add canonical FigureCanvas names.
Better documentation for print_figure refactoring.
Merge pull request #2588 from pwuertz/print_refactor

Refactor mechanism for saving files.