Add optional button to toggle preview

[tghelew]

Hi, I've made some changes in three main js files to optionally add a button in the help button group. This button show or hide the preview panel through jQuery.fadeToggle() function. I tried to keep your coding style and use of tabulation as much as possible. I made a few tests on two browsers namely:

• Chromium (Windows & Linux UA)
• dwb

Regards, Tlux

PS: Let me know if you want me to regenerate the .min version of the combined js.