UI: rewrite statusbar message function

* use the public method |XULBrowserWindow::setJSStatus| instead of setting a
  label attribute directly. It's safe.
* rename from |update| to |message|
UI: improve former message cleanup
UI: make |OverLink| handler

* use a flag instead of replacing a empty function to disable |setOverLink|
UI: make convenient function
UI: delete unused function
UI: add tip for redirect URL
UI: add comments
MouseGesture: cancel gestures on XUL popup