Fix editor markdown tab

Fixes #3813, reference #3810

* Add showPreview action to controller
* Implement action and bind attribute
* Remove the now pointless

![test](https://cloud.githubusercontent.com/assets/4590364/3977943/f52f1004-2840-11e4-9d72-27f4e3e18473.gif)


Could this get renamed to "toggleIsPreview" or "togglePreviewing" or some such?

And, would you add the property isPreview to the editor-base-controller and set its initial state to false?