API: Allow extensions to specify "useless" query pages

Some query pages are not useful for ApiQueryQueryPage, as they require
arguments that cannot be specified with that interface. Currently, the
list of such pages is in a private static variable in ApiQueryQueryPage;
this moves it to a global so extensions can add to the list.

Change-Id: Ieccd2e84803dc1de4e26cba85d6cdda13916647f