GeglMetadataStoreparse_value

This method is called to optionally parse image file metadata prior to setting metadata variables in the GeglMetadataStore. If no parser is available it returns FALSE and the registered mapping is used. If a parser available it should set one or more metadata variables using gegl_metadata_store_set_value() and return TRUE. Note that the parser MUST return TRUE even if setting individual values fails. The default method checks if a signal handler is registered for the parse-value signal with the variable name as the detail parameter. If a handler is registered it emits the signal with the file metadata provided as a GValue and returns TRUE otherwise FALSE.