MONACO, April 8 - While the television here plays vintage clips of the late Prince Rainier's storybook marriage to the American actress Grace Kelly, the unspoken question hanging in this tiny principality's crisp Mediterranean air is who the next princess will be. Prince Rainier's enigmatic son, Albert, who assumed Monaco's throne upon his father's death on Wednesday, is unmarried and has shown no public inclination to produce a royal heir. Choosing a bride to replace his mother, who died in a car crash 23 years ago, is an almost insurmountable task, people in the principality say. "People will be very...