ConfigOptions: GUI type as enum, not string.

Fixing compilation error in the new Platform code.
Fixing one issue in FDM support after splitting the top/bottom
interface layers.

src/libslic3r/Config.hpp

@@ -1441,6 +1441,24 @@ private:
 class ConfigOptionDef
 {
 public:
+    enum class GUIType {
+        undefined,
+        // Open enums, integer value could be one of the enumerated values or something else.
+        i_enum_open,
+        // Open enums, float value could be one of the enumerated values or something else.
+        f_enum_open,
+        // Color picker, string value.
+        color,
+        // ???
+        select_open,
+        // Currently unused.
+        slider,
+        // Static text
+        legend,
+        // Vector value, but edited as a single string.
+        one_string,
+    };
+
 	// Identifier of this option. It is stored here so that it is accessible through the by_serialization_key_ordinal map.
 	t_config_option_key 				opt_key;
     // What type? bool, int, string etc.
@@ -1524,7 +1542,7 @@ public:
     // Usually empty. 
     // Special values - "i_enum_open", "f_enum_open" to provide combo box for int or float selection,
     // "select_open" - to open a selection dialog (currently only a serial port selection).
-    std::string                         gui_type;
+    GUIType                             gui_type { GUIType::undefined };
     // Usually empty. Otherwise "serialized" or "show_value"
     // The flags may be combined.
     // "serialized" - vector valued option is entered in a single edit field. Values are separated by a semicolon.