uterm: input: add support for multiple keysyms

uterm: input: add support for multiple keysyms

xkbcommon supports reporting multiple keysyms per key-event. There is no
keymap which uses this, yet. However, this feature is not meant to enhance
performance by reducing the number of calls into xkb, but instead multiple
keysyms are to be handled as one big keysym.
But there are no examples, yet, so we cannot actually perform any
conversions on them. But we can add the infrastructure for it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>