j4status-plugin.h: Rename "self" to "section"

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
j4status-plugin.h: Reorder

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
J4statusSection: Rework API

Now plugins must set immutable data before inserting the section in the
list.
State data can now only be changed after inserting the section in the
list.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
J4statusSection: Properly remove the section from the list on free

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