``NavigationToolbar2.save_figure`` now returns filepath of saved figure
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``NavigationToolbar2.save_figure`` function may return the filename of the saved figure.

If a backend implements this functionality it should return `None`
in the case where no figure is actually saved (because the user closed the dialog without saving).

If the backend does not or can not implement this functionality (currently the Gtk4 backends
and webagg backends do not) this method will return ``NavigationToolbar2.UNKNOWN_SAVED_STATUS``.
