WASHINGTON - A dream for 100 years, the National Museum of African American History and Culture promises to become an instant favorite when it opens Sept. 24, its soaring spaces and magical views of the Mall a fitting setting for its tale of African-American history and achievement. Museum officials offered a sneak peek last week at the 400,000-square-foot museum, the 19th of the Smithsonian Institution, that’s next to the Washington Monument. President Obama is expected to cut the ribbon on the dramatic space, which features layers of galleries focused on slavery, segregation and the civil rights movement as well as...