Bug 900907 - [Keyboard] Update the pinyin IME layout with the UX specification

- optimize candidates rendering performance
- add candidate panel paging mechanism
- drop traditional chinese support
Merge pull request #11689 from luke-chang/900907_update_pinyin_ux_merged

Bug 900907 - [Keyboard] Update the pinyin IME layout with UX specification. r=yxl, rudylu.