Buttons: NgbRadioLabel directive is too "agressive"

[pkozlowski-opensource]

The NgbRadioLabel as implemented today prevents checkbox demo from working correctly. It is implementation is very simple:

@Directive({selector: 'label.btn', host: {'[class.active]': 'checked'}})
export class NgbRadioLabel {
  @Input() checked: boolean;
}

But if we move to NgModules this directive gets activated on all labels with the btn class (quite common scenario). And it can mess up scenarios like this:

  <label class="btn btn-primary" [class.active]="model.left">
    <input type="checkbox" [(ngModel)]="model.left"> Left (pre-checked)
  </label>

In this scenario [class.active]="model.left" will be overridden by the NgbRadioLabel directive and will never take effect. We can't remove active label when the directive is not used / manipulated from outside.

[pkozlowski-opensource]

I've got a fix for it in 21fa9a2 but it is against the RC5 branch so will wait with opening a PR till we've got RC5 sorted out.