Makes a distinction between keypad arrows and non keypad arrows in the X11 backend

Previously, VK_LEFT was returned both for the left keypad arrow and for the left arrow which was particularly confusing as KeyEvent.VK_KP_LEFT already existed but wasn't used. There are a few remaining problems:
- this change should have an effect when the numerical lock is off but it works only when it is on (AZERTY keyboard)
- the wrong value is still returned when the numerical lock is off
- the same fix should be applied for XK_KP_End, XK_KP_Home, XK_KP_Page_Up and XK_KP_Page_Down but this time the constants are missing from com.jogamp.newt.event.KeyEvent

N.B: This fix is appropriate for all supported keyboard layouts. This bug was probably reproducible with QWERTY.