计算给定字符串中各个单词的出现次数并按出现次数降序排序。


字符串：Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book.
Lorem: 2, Ipsum: 2, the: 2, text: 2, of: 1, is: 1, simply: 1, dummy: 1, printing: 1, and: 1, typesetting: 1, industry.: 1, has: 1, been: 1, industry's: 1, standard: 1, ever: 1, since: 1, 1500s,: 1, when: 1, an: 1, unknown: 1, printer: 1, took: 1, a: 1, galley: 1, type: 1, scrambled: 1, it: 1, to: 1, make: 1, specimen: 1, book.: 1.（以出现次数从高到低排序）