Samsung grabbed tablet market share from Apple during the first quarter of this year, but the iPad maker retained the top spot in an otherwise slowing tablet market. Worldwide tablet shipments totaled 50.4 million units during the quarter, increasing by just 3.9 percent compared to the same quarter a year ago, according to an IDC study released Thursday. The shipments rose at a slower rate than expected by IDC, which said the tablet category could struggle this year.