uterm: input: add flag to input-events marking them as handled

uterm: input: add flag to input-events marking them as handled

If multiple handlers are called on the same input-event, we must notify
handlers whether the event was already handled by a previous callback. We
push this decision to the handlers by allowing them to modify the
"handled" flag for an input event.

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