Make gtk3 full_screen_toggle more robust against external changes.

[anntzer]

The fullscreen state can also change via the window manager, so read the
state instead of trying to keep an internal flag. All other backends
already use a similar strategy.

(self.window is a GtkWindow, whereas self.window.get_window() is a
GdkWindow...)

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).