BUG: Right click on LeftAndMain menu caused CMS preview window to open

The onclick event for LeftAndMain menu links didn't check if the click
was left or right, meaning that right click events could trigger the
function for loading split view mode in some browser/os combinations.
Merge pull request #1052 from kinglozzer/leftandmain-preview-click-bug

BUG: Right click on LeftAndMain menu caused CMS preview pane to open