Allow users to set the waiting time when displaying a preview image using an external viewer

[seisman]

Description of proposed changes

See #1061 for the detailed issue report and discussions.

This PR adds the waiting parameter to the launch_external_viewer function. The default value is 0, so no waiting by default.

As mentioned in #1061 (comment), the launch_external_viewer function is used at two places, one in the Figure.show() function, another in the Figure.savefig() function.

• The function call in the Figure.show() opens the preview image in the temporary directory, thus we need to set the waiting time to a non-zero value (waiting=0.5 works for most users) so the external viewer can open the preview image before the PyGMT session exits.
• The function call in the Figure.savefig() opens the image that was already saved in the current directory or elsewhere, and won't be deleted when the PyGMT session exits. Thus there is no need to set the waiting time here.

Fixes #1061.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

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