x11: add XKB protocol keymap and state creation support

x11: add XKB protocol keymap and state creation support

These are function to create an xkb_keymap directly from XKB requests
to the X server. This opens up the possibility for X clients to use
xcb + xcb-xkb + xkbcommon as a proper replacement for Xlib + xkbfile for
keyboard support.

The X11 support must be enabled with --enable-x11 for now.
The functions are in xkbcommon/xkbcommon-x11.h. It depends on a recent
libxcb with xkb enabled. The functions are in a new libxkbcommon-x11.so,
with a new pkg-config file, etc. so that the packages may be split, and
libxkbcommon.so itself remains dependency-free.

Why not just use the RMLVO that the server puts in the _XKB_RULES_NAMES
property? This does not account for custom keymaps, on-the-fly keymap
modifications, remote clients, etc., so is not a proper solution in
practice. Also, some servers don't even set it. Now, the client just
needs to recreate the keymap in response to a change in the server's
keymap (as Xlib clients do with XRefreshKeyboardMapping() and friends).

Signed-off-by: Ran Benita <ran234@gmail.com>