Get rid of the xcb-util-keysyms dependency in favor of libxkbcommon{,-x11}

Currently, we use a mixture of xcb-util-keysyms calls and libxkbcommon{,-x11} calls. As @bluetech explains, we should be able to switch entirely to libxkbcommon{,-x11}:

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It should be possible to remove the xcb/util/keysyms dependency and the all core protocol keyboard-related requests/events entirely. If you try that and there is something that you can't do, I'll be happy to help.

It should also be possible to keep the xkb_state updated with xkb_state_update_mask in the XKB_STATE_NOTIFY event handler; then you can query the xkb_state instead of interpreting the ->state field of the KeyPress event at all.

It should also be possible :) to use xkb_keysym_to_utf32 instead of i3-input/keysym2ucs.c and related files (internally it uses the same table mostly).
"""