Two prominent commercial real estate development and management firms in Omaha are merging, confident in the belief that the recession-battered industry is on the mend. The merger of Seldin Co., an 88-year-old family-owned firm, and Coldwell Banker World Group will create World Group Commercial Real Estate, which will control more than 3 million square feet of retail, office and industrial space in the Omaha metro area, said executives of the two firms.