Fix for Issue 1470: On OS X TextField deletes symbols from the right side

ref: https://code.google.com/p/libgdx/issues/detail?id=1470

This pull request fixes the behavior of the backspace key in TextField on OS X.

On OS X, the KeyEvent.keyChar field on a Backspace press contains 127, not 8 as expected. This is what is reported by org.lwjgl.input.Keyboard.getEventCharacter().  

org.lwjgl.input.Keyboard.getEventKey() is distinguishing correctly between Backspace and Delete, so keyCode can be used to fix keyChar.