Police in Bucks County have the dragnet out for a bank robber.   Investigators say the bandit walked out of the First Niagara bank branch on North Main Street in Sellersville with an undisclosed amount of cash after handing a teller a demand note.   The robber indicated that he had a gun but did not display it.   Witnesses say he fled on foot and describe him as a white, balding man in his 50s wearing a t-shirt with an Italian flag on the back.