By Mark Felsenthal GOREE ISLAND, Senegal (Reuters) - Almost four centuries after Africans started being shipped to North America as slaves, the first U.S. president of African ancestry on Thursday visited an infamous embarkation point for those destined for lives in chains. In his first - and, many Africans say, long-overdue - extended tour of the continent since entering the White House, President Barack Obama focused on political and economic issues while also recalling a painful chapter in Africa's and America's past. ...