Correct handling of multimedia keypresses

http://trac.xbmc.org/ticket/13585 reports that on some laptops multimedia keypresses are having unintended affects. This is because on some keyboards (it isn't clear exactly which) the multimedia keys send scan codes used by different keys e.g. the mute key sends the same scancode as the "d" key. This means that (on Windows) XBMC sees the WM_APPCOMMAND message from the mute key, but then a rogue "d" keypress as well. This has started mattering since new key mappings were added to support the PVR functions.

This commit is a quick fix that stops XBMC processing the scan code for multimedia keys. This should do for Frodo, though in the longer term we need to look at exactly what scancode processing is required as it isn't clear why the current code is needed.