It's unnecessary, and only adds visual noise; we have been fairly
inconsistent in the past, but the semi-colon-less version clearly
dominates in the code base.
https://bugzilla.gnome.org/show_bug.cgi?id=669355
If we have an input parameter (or return value) we need to use (nullable).
However, if it is an (inout) or (out) parameter, (optional) is sufficient.
It looks like (nullable) could be used for everything according to the
Annotation documentation, but (optional) is more specific.
Use #GVariant instead of GVariant.
g_notification_add_button_with_target,
g_notification_set_default_action_and_target:
Replace 'format_string' with 'target_format'.
g_notification_set_default_action_and_target_value:
Remove paragraph that apparently had been accidentally copied from
g_notification_set_default_action_and_target.
https://bugzilla.gnome.org/show_bug.cgi?id=727123
