better checkbox alignment in catalogtree

Before:
![capture decran 2013-09-02 a 15 59 01](https://f.cloud.github.com/assets/21686/1067750/38c204a2-13d9-11e3-982c-8df55c43a1e7.png)

After: 
![capture decran 2013-09-02 a 16 05 40](https://f.cloud.github.com/assets/21686/1067751/38dbcd9c-13d9-11e3-829a-60b1e754c2b7.png)