recent xcb_key_symbols_get_keycode() now returns a pointer

Keep old API (as present in Debian Lenny for example) as requested by nefrir
GetModifier() is broken when using the new API, it might be broken as
well when using the old API.

Documents / source code of functions used in GetModifier() are very
welcome, libxcb only ships XML descriptions and the generated source
code is very unreadable.