Create a FocusFrame control

If the app creates a custom control, and wants it to have a focus frame that matches the  style, there is currently no API that lets him do that. 

Like widgets, there should be a FocusFrame control in controls. This will enable an app to create a native looking focus frame that it can resize, move on top of, and inside, it's own custom controls. 

We should consider letting the FocusFrame just be a thin proxy/API for the actual focus frame, since it might be implemented as a singleton (to reduce memory usage etc).

Example:

MyCusomControl {
   id: fluffy
   FocusFrame {
      visible: fluffy.hasActiveFocus
      aroundItem: fluffy // the target item can be a child etc
      offsetLeft: 0 // adjust position relative to fluffy.x
      ...
      radius: 5 // some custom control can be round
   }
}