contains the names of blocks that contain lists of button names
buttons: {
    "undo": ["undo","redo"],
    "font": ["font-family","font-size","font-weight","font-style",
    "text-decoration","color","background","borders"],
    "align": ["text-align","vertical-align","wrap","span"],
    "format": ["format"]
}
Note that this property works only with the default toolbar.