Fix long category name overflow in vertical tabs

The bug was reported in https://github.com/modxcms/revolution/issues/11728, looked like that:

![bildschirmfoto 2014-07-20 um 17 43 41](https://cloud.githubusercontent.com/assets/445501/3637891/63d023e0-1027-11e4-8b3b-a64fbfe5260e.png)

with the proposed fix it look like that:

![bildschirmfoto 2014-07-20 um 17 59 42](https://cloud.githubusercontent.com/assets/445501/3637892/7ec9efa0-1027-11e4-8a31-865b9d139496.png)

@Mark-H: this could also solve the problem you had here: #11634 (still would love the vtabs in the user group edit panel)