rename variable for readability
MoveTabToWindow: add tooltip for private window
Util: use original code of namespace fixer for xpath

* see http://nanto.asablo.jp/blog/2008/12/11/4003371
* my modified code was buggy to detect element names
rename |TimerHandler| to simple |Timer|
unify functions for preferences into a handler

* passing |null| to a setting function clears a preference no longer.
  use |Prefs.clear|