Make CGUIInfoManager::Register return a pointer to the InfoBool object
rather than an index into its cache of such objects.

Expression infobools now determine whether any of their leaves are dependent
upon the list item by examining the corresponding leaf object directly, since
it now has the pointer returned from CGUIInfoManager::Register, rather than
relying on CGUIInfoManager::Register returning it via the 3rd argument (now
removed). The InfoExpression class doesn't have access to protected members
of InfoBool though, so m_listItemDependent is now a public member.