Add support for modmap None (#291)

Unlike current xkbcommon, X11’s xkbcomp allows to remove entries in
the modifiers’ map using “modifier_map None { … }”.

“None” is translated to the special value “XkbNoModifier” defined in
“X11/extensions/XKB.h”. Then it relies on the fact that in "CopyModMapDef",
the following code:

    1U << entry->modifier

ends up being zero when “entry->modifier” is “XkbNoModifier” (i.e. 0xFF).
Indeed, it relies on the overflow behaviour of the left shift, which in
practice resolves to use only the 5 low bits of the shift amount, i.e.
0x1F here. Then the result of “1U << 0xFF” is cast to “char”, i.e. 0.

This is a good trick but too magical, so in libxkbcommon we will use
an explicit test against our new constant XKB_MOD_NONE.

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