Sorting: fix a bug.

During sorting, emoticons should not be add/removed by clicking. Add a
banner to notify user to save sorting result before add/remove.
Sorting: layout fixes.

Improve shaking animation.
Change a page button icon.
Polish codes in options.js .

Hoist some static functions.
Storage related functions cannot be hoisted due to variable scope, which
is difficult to couple with.
Combine image & text storage functions into one.