Before V4:
* GdaDataModelIter directly changing GdaHolder's values
* API conventions overview and corrections
* GdaDataModel error codes
* review GdaDataModelAccess and error reporting
* gda_data_model_signal_emit_changed() => gda_data_model_changed()

tools/command_exec: command->arguments_delimiter_func should be a function which does not
                    remove double quotes, except for some commands
libgda/gda-meta-struct.c: gda_meta_struct_get_db_object() should use the same convention as _complement()
                          about case sensitiveness.
