gets the current active frame of this container
(not of any other available supplier)
This may be the frame itself. The active frame is defined as
the frame which contains (recursively) the window with the focus.
If no window within the frame contains the focus, this method
returns the last frame which had the focus. If no containing
window ever had the focus, the first frame within this frame is
returned.
In general this method first calls the method XFramesSupplier::setActiveFrame()
at the creator frame with this as the current argument. Then it broadcasts
the FrameActionEventFrameAction::FRAME_ACTIVATED.
Note: Given parameter Frame must already exist inside the container
(e.g., inserted by using XFrames::append())