Windows: Add numpad keys and VK_APPS

This brings Windows keyboard input up to speed with the other platforms. Instead of treating the navigation keys on the numpad the same as the navigation cluster, it uses the enhanced/extended flag in lParam to tell them apart. [According to MSDN,](http://msdn.microsoft.com/en-us/library/windows/desktop/ms646280.aspx) the numpad navigation/arrow keys have the extended flag clear and the numpad enter key has the extended flag set.

VK_APPS was also missing, probably because of the confusing name (it's the menu key, VK_MENU is Alt.)

This slightly reorders the logic in `translate_key_input`, but it think it looks a little better, now that there are (potentially) two calls to `lookup_keymap_table`.