Sharing role overlay: fix wrong javascript selector for ftw.permissionmanager

The problem was that all <th> tags were sliced (first removed), but this included both tables.
This change makes slicing per table and separate setups so that the name column of the second
table is not linked.

Also refactored overlay setup into separate function.

![bildschirmfoto 2014-05-08 um 09 10 50](https://cloud.githubusercontent.com/assets/7469/2912760/38e8cfaa-d680-11e3-8315-c2e2ed981cc7.png)
@maethu @deiferni