libj4status-plugin: Add short value API

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
i3bar: Add short value support

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
flat: Use \a for bell

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
gitmodules: Update libnkutils

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>