[NUI] Add attached bindable property CheckBoxGroup.IsGroupHolder to the View. (#3426)

[NUI] Add attached bindable property CheckBoxGroup.IsGroupHolder to the View. (#3426)

In NUI, the parent View that holds checkboxes does not related with CheckBoxGroup.
Which means user must create CheckBoxGroup manually and then add all checkbox to the group.

```
var check1 = new CheckBox();
var check2 = new CheckBox();
var check3 = new CheckBox();

parent.Add(check1);
parent.Add(check2);
parent.Add(check3);

var group = new CheckBoxGroup();
group.Add(check1);
group.Add(check2);
group.Add(check3);
```

In this structure, user can not make a group in XAML code.
This patch introduce attached property to the View for the CheckBoxGroup:
```
<View CheckBoxGroup.IsGroupHolder="True">
  <CheckBox Text="Check1"/>
  <CheckBox Text="Check2"/>
  <CheckBox Text="Check3"/>
</View>
```

And then the user can access check box group from the View using GetCheckBoxGroup(view).

Signed-off-by: Jiyun Yang <ji.yang@samsung.com>