UI: display message at anytime

* regression by commit: 5338535
  |XULBrowserWindow::setJSStatus| doesn't display a given status when the page
  load status is showing
UI: optimize overlink functions

* check |url| to avoid the needless process for a empty |url|
* fix undefined |linkState| when an invalid url
* it is enough to set the attribute of link state in |setOverLink|
  therefore |linkState| became a local variable