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Manage categories and handle database communication.
Only one instance of this class should exist for one database connection, i.e. one instance of GALGallery. This is assured inside __construct(). Most work is done by inherited methods from GALTable.
This constructor shouldn't be called by a library user, the representations for the used tables are automatically created by the GALGallery object.
Checks if an instance of GALCategories already exists in the specified GALGallery. If this is the case, it throws an GAL_ECategory. If not, it reads necessary information for further initialization from $glb.