test: add "how to type" demo program

test: add "how to type" demo program

The program takes a unicode codepoint and an RMLVO and prints out all
key + modifier combinations that would result in that codepoint.

The program was written to exercise the new
xkb_keymap_key_get_mods_for_level() function. It's handy and can be
extended in several ways, but enough for now.

Example:

    $ ./build/how-to-type -l us,il,ru 0x41 | column -ts $'\t'
    keysym: A (0x41)
    KEYCODE  KEY NAME  LAYOUT#  LAYOUT NAME   LEVEL#  MODIFIERS
    38       AC01      1        English (US)  2       [ Shift ]
    38       AC01      1        English (US)  2       [ Lock ]
    38       AC01      2        Hebrew        2       [ Shift ]
    38       AC01      2        Hebrew        2       [ Lock ]

    $ ./build/how-to-type -l de -v neo 0x3b6 | column -ts $'\t'
    keysym: Greek_zeta (0x7e6)
    KEYCODE  KEY NAME  LAYOUT#  LAYOUT NAME     LEVEL#  MODIFIERS
    56       AB05      1        German (Neo 2)  4       [ Shift Mod5 ]
    56       AB05      1        German (Neo 2)  4       [ Shift Mod2 Mod3 Mod5 ]
    56       AB05      1        German (Neo 2)  4       [ Shift Lock Mod5 ]
    56       AB05      1        German (Neo 2)  4       [ Lock Mod2 Mod3 Mod5 ]

Signed-off-by: Ran Benita <ran@unusedvar.com>