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.hpp

@@ -702,6 +702,8 @@ public:
     std::string  subtask_id_;
     std::string  job_id_;
     std::string  last_subtask_id_;
+    // note: printer-agent-supplied thumbnail url, empty when the agent supplies none.
+    std::string  m_agent_thumbnail_url;
     BBLSliceInfo* slice_info {nullptr};
     boost::thread* get_slice_info_thread { nullptr };
     boost::thread* get_model_task_thread { nullptr };