Limit available plugins in the editor

If I have this setting : 

```python
CMS_PLACEHOLDER_CONF = {
    'home_abstract': {
        'plugins': ['TextPlugin',],
        'text_only_plugins': ['LinkPlugin',],
    },
}
```
And I have many other text compatible plugins (eg : FilerFilePlugin and FilerImagePlugin), we have all text compatible plugins in the CKEditor plugins dropdown and not only plugins listed in `text_only_plugins`.
This is because `plugin_pool.get_text_enabled_plugins` and `plugin_pool.get_all_plugins` require a slot name and not a placeholder instance to retrieve the placeholder configuration. As it can not find the conf, the available plugins list return ALL plugins.