consider making sun.awt.CausedFocusEvent functionality public

Currently, the sun.awt.CausedFocusEvent class is used internally as a runtime value of a focus event. The class adds a "cause" field to java.awt.FocusEvent which describes an action caused the event. The set of the "cause" values is defined as below which is pretty mnemonic:

        UNKNOWN,
        MOUSE_EVENT,
        TRAVERSAL,
        TRAVERSAL_UP,
        TRAVERSAL_DOWN,
        TRAVERSAL_FORWARD,
        TRAVERSAL_BACKWARD,
        MANUAL_REQUEST,
        AUTOMATIC_TRAVERSE,
        ROLLBACK,
        NATIVE_SYSTEM,
        ACTIVATION,
        CLEAR_GLOBAL_FOCUS_OWNER,
        RETARGETED

This information may give a client an ability to make its response to a focus event more specific.

Also, in conjunction with it, we should consider adding appropriate focus request methods to a component:

public boolean requestFocus(CausedFocusEvent.Cause)
public boolean requestFocusInWindow(CausedFocusEvent.Cause)