PreviewRail

PreviewRail adds an imitation rail to the preview page, which allows you to view the contents of the page not in full width, taking into account the rails. PreviewRail2 adds alternative button in bottom-right place.
== ==
Alternative button without bugs.
== Known bugs ==
* Adds button in ve-mode only when open it from ve-sourse
* Adds duplicate button in ve-sourse preview each time you click on the preview button.