api: deprecate XKB_MAP_COMPILE_PLACEHOLDER, and use KEYMAP instead of MAP

api: deprecate XKB_MAP_COMPILE_PLACEHOLDER, and use KEYMAP instead of MAP

The PLACEHOLDER was not meant to be used, but c++ doesn't like passing 0
to enums, so it was used. For this reason we add all the NO_FLAGS items,
so the PLACEHOLDER shouldn't be used anymore.

Second, XKB_MAP is the prefix we used ages ago, KEYMAP is the expected
prefix here. So deprecate that as well.

The old names may still be used through the xkbcommon-compat.h header,
which is included by default (no need to include directly).

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