Add option avoid QtWebEngineWidgets

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including CHANGELOG.rst for all changes

This PR adds the possibilities to avoid importing PyQt5.QtWebEngineWidgets and disable the HTML help (Plain text help is still available). Here we implement:

• a new rcParams key 'help_explorer.use_webengineview', which defaults to True
• the new command-line options psyplot --webengineview and psyplot --no-webengineview to override the rcParams