Gives keyboard input focus to this item. The focusReason argument will be passed into any focus event generated by this function; it is used to give an explanation of what caused the item to get focus.
Only enabled items that set the ItemIsFocusable flag can accept keyboard focus.
If this item is not visible, not active, or not associated with a scene, it will not gain immediate input focus. However, it will be registered as the preferred focus item for its subtree of items, should it later become visible.
As a result of calling this function, this item will receive a focus in event with focusReason. If another item already has focus, that item will first receive a focus out event indicating that it has lost input focus.