Return filename from save_figure

[Zybulon]

Closes #27744

PR summary

This PR adresses issue #27744.
save_figure functions from the NavigationToolbar return the filename of the saved figure.
If no figure is save then it returns None.
For GTK4 backend and Web backend I could not get the filename so the function still returns None.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

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[tacaswell]

We can probably use a closure / nonlocal with the gtk response callback, but I'm not sure if dialog.show() blocks until the window is closed or return immediately. If it blocks then we can get the the filename out of that one too.

I agree we the webagg one looks like a big lift because I don't think we currently have a response back over the websocket. Even if we added that, it is not super clear it would be useful as the filesystems that the client (in a browser possibly on a different machine) is in general not the filesystem that the server sees. There is also a good case we should not leak anything about the client file system back to the server.

[tacaswell]

If we want to differentiate between "I did not save a file" and "I can not tell you if a file was saved" we could return a different sentinel in the "I did not save a file case". The simplest thing (from an implement ion point of view...typing this proposal would be hard) would be to add NO_FILE_SAVED = object() as a class attribute on NavigationToolbar2 and return that if no file is saved.

[Zybulon]

That is a good idea. I'll do that.

[Zybulon]

Usually the FileDialogs (from Qt, Gtk etc...) return None when no file is selected. To be consistent with this behavior, I propose returning None when no file is saved and a class attribute UNKNOWN_SAVED_STATUS = object() when the status is unknown. What do you think ?

[Zybulon]

Hello,
Sorry for the delay. I think the pull request is ready for review when someone has time to do so.