Add i3 focus event input plugin

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
i3focus: Handle section insertion failure

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
i3focus: Limit to the focus change

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
libj4status: Merge to libj4status-plugin

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
i3focus: Migrate from j4status

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
HACKING.md: Add i3focus example

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