GUI: rename "system" to "chip"

"system" made sense when Furnace was a .dmf tracker and had
compound setups like Genesis (YM2612+SN)

however, it doesn't make too much sense now when compared to "chip"

src/gui/debug.cpp

@@ -334,7 +334,7 @@ void putDispatchChan(void* data, int chanNum, int type) {
       break;
     }
     default:
-      ImGui::Text("Unknown system! Help!");
+      ImGui::Text("Unimplemented chip! Help!");
       break;
   }
 }