support mouse hover in locator

User can do mouse hover in locator as follows, added a "hover" boolean
value as locator test parameter.
{
  "controller":"locator",
  "params": {
    "value":"#y-main-nav li:nth-child(7) a span",
    "hover":true,
    "waitForElement":"#y-main-nav li:nth-child(7) .nav-sub li:nth-child(4)"
  }
}

waitForElement is optional if user does not wait for other element after mouse hover is done.