TextInputControlBehavior.lastEvent leaks memory

When using TextFields on Popups, KeyEvents will not only target the focused TextField on the Popup, but the final event target is the focused UI control on the main window. Because TextInputControlBehavior.lastEvent holds on to the last received KeyEvent, the TextField will keep the currently focused UI control on the main window in memory.

This bug occurs on Java 7 (jdk1.7.0_72 win x64), but not on Java 8 (jdk1.8.0_40-ea win x64).