Replace fake-enum printer agent dropdown (#121)

A dedicated PrinterAgentChoice field
reads rows straight from the live
agent registry and stores the agent
id string, replacing the fake-coEnum
index mapping. The field moves to
TabPrinter and registers with the
searcher so UnsavedChanges renders
it; the PhysicalPrinterDialog copy
and its update hook are removed
(#125). switch_printer_agent now
resolves ids via
resolve_printer_agent_id.

src/slic3r/GUI/PhysicalPrinterDialog.hpp

@@ -60,7 +60,6 @@ public:
 
     void        update(bool printer_change = false);
     void        update_host_type(bool printer_change);
-    void        update_printer_agent_type();
     void        update_preset_input();
     void        update_printhost_buttons();
     void        update_printers();