When editing the order of class attributes on multiple browser windows/tabs using the up/down arrows, the attribute IDs get mixed up.

Steps to reproduce

1. Open eZ Publish on two browser tabs and edit the 'folder' class (for example) on both;
2. On the first tab, move the first attribute two positions down, and without clicking on OK, switch to the second browser tab, move the third attribute two positions up and now click OK. You'll notice that the IDs of each attribute have changed.