First attempt for searchable QRadioElement

This isn't a final version. I don't even know if it's anywhere near what you were thinking of, but this is the attempt I made and it does work for me. 

I do know that for it to be accepted it should probably be refactored here and there (to be more generic or to conform to your style).

To make use of this feature:

```objective-c
QRadioElement *radioElement = [[QRadioElement alloc] initWithDict:dict selected:selectedIdx title:@"title"];

radioElement.searchable = YES;
```

I'd like to know if it's a direction you like and hear some feedback, then I'd make required changes (I might not be aware of all QD's features that might be affected by this change).