Use r/o checkbox for enabled/active, make columns sortable

The custom renderer for 'active' was supposed to add a tooltip with
the reason why it's not active, but that did not work before.

![capa-ui](https://f.cloud.github.com/assets/169314/131487/88f5c308-705c-11e2-8b7d-ae3ebad3afc5.png)