Not correctly handling AppIndicators' status changes

The panel's Status Tray plugin is not at all correctly handling AppIndicators' status changes because it only detects whether status is Passive or not, cf. sn_item_status_is_exposed(). But status is a tri-state value; it may not only be Passive or Active but might be NeedsAttention as well (according to enum AppIndicatorStatus). Only in the case if status equals NeedsAttention, the attention icon and its description shall be shown. With your implementation, the attention icon is displayed as soon as it is set by receiving the signal NewAttentionIcon and status not being Passive (cf. property exposed), which lets any application like Solaar always display the attention icon, also if status remains to just be Active all the time. But then the shown icon shall be the regular instead of the attention icon (according to StatusNotifierItem's documentation for instance).