Improve GtkBuilder error reporting

Make GtkBuilder report an error when it encounters a duplicate id,
instead of segfaulting later on

gtk/gtkbuilder.h

@@ -52,7 +52,8 @@ typedef enum
   GTK_BUILDER_ERROR_INVALID_TAG,
   GTK_BUILDER_ERROR_MISSING_PROPERTY_VALUE,
   GTK_BUILDER_ERROR_INVALID_VALUE,
-  GTK_BUILDER_ERROR_VERSION_MISMATCH
+  GTK_BUILDER_ERROR_VERSION_MISMATCH,
+  GTK_BUILDER_ERROR_DUPLICATE_ID
 } GtkBuilderError;
 
 GQuark gtk_builder_error_quark (void);