app: maintain common settings when editing Brightness-Contrast/Levels as Levels/Curves

When converting Brightness-Contrast to Levels, and Levels to
Curves, make sure to copy the common settings as well as the
operation-specific settings.

(cherry picked from commit 987447f18f)

app/operations/gimpbrightnesscontrastconfig.c

@@ -181,6 +181,10 @@ gimp_brightness_contrast_config_to_levels_config (GimpBrightnessContrastConfig *
 
   levels = g_object_new (GIMP_TYPE_LEVELS_CONFIG, NULL);
 
+  gimp_operation_settings_config_copy_base (GIMP_CONFIG (config),
+                                            GIMP_CONFIG (levels),
+                                            0);
+
   brightness = config->brightness / 2.0;
   slant = tan ((config->contrast + 1) * G_PI_4);
 

app/operations/gimplevelsconfig.c

@@ -695,6 +695,10 @@ gimp_levels_config_to_curves_config (GimpLevelsConfig *config)
 
   curves = g_object_new (GIMP_TYPE_CURVES_CONFIG, NULL);
 
+  gimp_operation_settings_config_copy_base (GIMP_CONFIG (config),
+                                            GIMP_CONFIG (curves),
+                                            0);
+
   curves->linear = config->linear;
 
   for (channel = GIMP_HISTOGRAM_VALUE;