Bring Moonraker device panel to feature parity

The monitor panel showed wrong or missing
data for Moonraker printers, and its
controls did nothing.

Push payload now carries layer number and
total layers. Remaining time replaces the
wrong total_duration - print_duration
formula. The chamber light toggle maps to
Klipper SET_PIN / SET_LED, and pause,
resume and stop post to
/printer/print/{action}. Task thumbnails
resolve via /server/files/thumbnails onto
a new MachineObject thumbnail url.

Filament sync switches to pull mode so the
agent is queried on demand.

Not compiled or run.

src/slic3r/GUI/DeviceManager.cpp

@@ -3470,6 +3470,9 @@ int MachineObject::parse_json(std::string tunnel, std::string payload, bool key_
                             this->task_id_ = jj["task_id"].get<std::string>();
                         }
 
+                        if (jj.contains("thumbnail_url") && jj["thumbnail_url"].is_string())
+                            m_agent_thumbnail_url = jj["thumbnail_url"].get<std::string>();
+
                         if (jj.contains("job_attr")) {
                             int jobAttr = jj["job_attr"].get<int>();
                             jobState_ =  get_flag_bits(jobAttr, 4, 4);