Convert usergroup tree to usergroup grid in ACL page

Convert usergroup tree to tabbed usergroup grids in ACL page to handle huge amount of groups/members.

From this:
![modx-tree-usergroup](https://cloud.githubusercontent.com/assets/308524/3947185/86f193a8-2685-11e4-807a-89eada7601ea.png)

To this:
![modx-grid-user-group-groups](https://cloud.githubusercontent.com/assets/308524/3947197/b9fda3fe-2685-11e4-9f20-240a9e6e924a.png)

Clicking the row will open closable tab which contains the members of the selected group.
![anonymous-group](https://cloud.githubusercontent.com/assets/308524/3947200/c51f75d2-2685-11e4-93ab-5fa391e29b04.png)

Unused attribute (and already declared in `MODx.grid.Grid`)