Fix order of import links in the cp toolbar.

#6276 

In this PR we order the "import" links according to the link titles to get the same order of the "add" tabs.
Regular ksort is not helping here because of the "biblio" key that is displayed using the "Publication" title.

Output:
![selection_281](https://cloud.githubusercontent.com/assets/4497748/3397412/7b7c66fa-fd1e-11e3-98b1-d5ffafbfc2aa.png)

=> usort callback function for ordering the links in alphabetical order.