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/libslic3r/Config.hpp

@@ -2273,6 +2273,8 @@ public:
         plugin_picker,
         // Raw JSON string value, edited through a dialog behind a button rather than in the row.
         plugin_config,
+        // PrinterAgentChoice
+        printer_agent_select,
     };
 
 	// Identifier of this option. It is stored here so that it is accessible through the by_serialization_key_ordinal map.