Fixes #3848: makes modal bg show/hide when swap style.

If one modal step is followed by a default style step, modal background
is hidden and if a default style step is followed by a modal step, the
modal background is shown.
Avoid to show joyride modal before needed.

This way, is up to the settings how the joyride modal will be shown
(with a simple show method or a fade effect).
Use fadeOut to hide joyride modal bg according to settings.
Merge pull request #5887 from michelts/joyride-modal-bg

Fixes #3848: show/hide modal background when swap style.