Replace 'multiple' property with 'isShowCheckbox' in NbOptionComponent to control it outside

[outstandyy]

Issue type

I'm submitting a ... (check one with "x")

• bug report
• feature request

Issue description

Current behavior:

NbOptionComponent has multiple property, which determines whether option should be with checkbox (multiple select).
After adding new autocomplete component (#1947) parent property may not be set, because NbOptionComponent can be used in NbAutocompleteComponent, which doesn't have multiple property. So now we have check on multiple property getter.

Expected behavior:

Control of multiple select mode outside component via isShowCheckbox.

Related code:

  get multiple() {
    // We check parent existing because parent can be NbSelectComponent or
    // NbAutocomplete and `miltiple` property exists only in NbSelectComponent
    return this.parent ? (<NbSelectComponent<T>>this.parent).multiple : false;
  }

Angular, Nebular

Angular: 8.0.0
Nebular 4.2.1