UI: implement Cards-style interface for plugin lists (squashed commits)

This commit change the list items layout for plugins to a more modern, card-based interface.
Here are some screenshots:

![screenshot_2014-02-11-16-59-37](https://f.cloud.github.com/assets/2505731/2138701/cd041768-9336-11e3-9c4c-54ce5fd484ff.png)
![screenshot_2014-02-11-16-59-46](https://f.cloud.github.com/assets/2505731/2138703/d087e658-9336-11e3-84ba-77182e1fd943.png)
![screenshot_2014-02-11-17-00-07](https://f.cloud.github.com/assets/2505731/2138706/d2b6dfd8-9336-11e3-83b1-47c0fb33b252.png)
![screenshot_2014-02-11-17-00-15](https://f.cloud.github.com/assets/2505731/2138707/d461f840-9336-11e3-8f76-3436eef832f7.png)

These are only cosmetic changes, so I made a pull request to ask your opinion on whether I should merge this or not.
@evilsocket, @tux-mind, what do you guys think ?