Added mutli-select feature in friendlist

Currently, `Tox-Qt-GUI` doesn't have multi-select support in the `friendsWidget`. With this patch, users will be able to select multiple friends and remove them. 
![blob3](https://f.cloud.github.com/assets/998311/2318451/117ff9b6-a368-11e3-9878-69776b911acf.png)