libgimp: properly free resources from the config.

Found by the definitely useful libgimp warnings:

> gimp_plug_in_destroy_proxies: ERROR: GimpPattern proxy with ID 13 was refed by plug-in, it MUST NOT do that!

libgimp/gimpbrushselectbutton.c

@@ -231,6 +231,7 @@ gimp_brush_select_button_get_brush_bitmap (GimpBrushSelectButton *self)
   result.mask_data = g_bytes_unref_to_data (mask_data, &mask_size);
   /* Discard any color data, bitmap is B&W i.e. i.e. depth one i.e. a mask */
   g_bytes_unref (color_data);
+  g_object_unref (brush);
 
   return result;
 }

libgimp/gimpgradientselectbutton.c

@@ -191,6 +191,8 @@ get_gradient_data (GimpGradientSelectButton *self,
       *sample_count = n_samples;
     }
 
+  g_object_unref (gradient);
+
   /* When result is true, caller must free the array. */
   return result;
 }

libgimp/gimppatternselectbutton.c

@@ -235,6 +235,8 @@ gimp_pattern_select_button_get_pattern_image (GimpPatternSelectButton *self)
                            &color_bytes);
   result.pixelels = g_bytes_unref_to_data (color_bytes, &pixelels_size);
 
+  g_object_unref (pattern);
+
   return result;
 }