not have to track all other possible filetypes. This should make it
easier to add new filetype backends without updating lots of places.
All GUI backends have the same base set of filetypes they support,
defined in FigureCanvasBase. Non-GUI backends, for the most part will
still only write to their own file format (and not do any
switch_backend magic.) All GUI backends (where possible) now generate
a list of file patterns for their file chooser dialogs, rather than
having hard-coded lists.
See FILETYPES for matrix of filetypes supported by each backend.
