ensure that deprecation message is hidden

On initially entering darkroom view, the deprecation warning message
is displayed on whichever tab is active in the right side panel even
if that tab is not the deprecated-modules tab.  Switching to any other
tab (and optionally back) hides the warning.

This commit ensures that the message is also hidden initially.

src/libs/modulegroups.c

@@ -2927,6 +2927,7 @@ void gui_init(dt_lib_module_t *self)
   d->current = dt_conf_get_int("plugins/darkroom/groups");
   if(d->current == DT_MODULEGROUP_NONE) _lib_modulegroups_update_iop_visibility(self);
   gtk_widget_show_all(self->widget);
+  gtk_widget_set_no_show_all(d->deprecated, TRUE);
   gtk_widget_set_no_show_all(d->hbox_buttons, TRUE);
   gtk_widget_set_no_show_all(d->hbox_search_box, TRUE);