Add button in Groups Tab allows to add a group if this one is disabled

Add button appears on top of Groups tab, and it's disabled. Even if it's disabled, we can add one and without to select any Directory.
Add and Delete should be hidden while no Directory is selected.

Be careful with https://issues.freepbx.org/browse/FREEPBX-22634
Maybe we  can Delete more than one group at once. As we can delete more than user from different directories, we imagine to do that for groups as well.