Util: add TODO comment
ListEx: small refactoring

* rename |aIsBuilt| to |aHasBuilt| which shows whether item has been built or
  not
* rename |getFaviconAndLastVisitedTime| to |getTimeAndFavicon| which fits in
  the order of callback parameters
* remove useless assignments of |action|
ListEx: remove extra items of recent history

* hidden items include the content of frames and redirected-from pages
* only toplevel pages which are shown in the URL bar are not hidden.
ListEx: use |PlacesUIUtils.ellipsis|
ListEx: minor fix

* rearrange local variables
* use |gBrowser.tabs| instead of deprecated |gBrowser.mTabs|
Util: put spaces into one in CSS