keymap: add Genius SlimStar 320

keymap: add Genius SlimStar 320

This keyboard have 16 "multimedia" keys plus a "4-way turbo scroll pad" which
is essentially a round up/down/left/right button. Unfortunately most of these
keys emit non-standard scancodes in a range 495-508 which does not make any
sense. I tried to remap those to the best of my knowledge.

Note the keyboard comes up as two event devices, second one is multimedia
keys, so I ended up adding ENV{ID_USB_INTERFACE_NUM}=="01" to avoid
unnecessary initialization of the "main" keyboard.