Expose [NS|UI]View.Subviews with NSArray variant

### Steps to Reproduce

1. Query NSView.Subviews
2. Observe that every time we call it, we marshal an array

### Expected Behavior
We should have an API endpoint that exposes an NSArray.

### Actual Behavior
There is none.

### Notes

Scenario:
```
NSView view = ...;
for (int i = 0; i < view.Subviews.Length; ++i) {
var subView = view.Subviews [i];
...
}
```

The above code would marshal the array twice, every time we iterate. Also, array length queries would have to marshal the array to managed, so simply doing:
```
bool hasChildren = view.Subviews.Count > 0;
```

Having an NSArray based overload would avoid that and help for performance with multiple subviews in the NSView.