inheritance of placeholder configuration

Issue #2705.

Exemple usage : 

```python
CMS_PLACEHOLDER_CONF = {
    'main': {
        'name': 'main content',
        'plugins': ['TextPlugin', 'LinkPlugin'],
        'default_plugins':[
            {
                'plugin_type':'TextPlugin', 
                'values':{
                    'body':'<p>Some default text</p>'
                },
            },
        ],
    },
    'layout/home.html main': {
        'name': u'main content with FilerImagePlugin and limit',
        'inherit':'main',
        'plugins': ['TextPlugin', 'FilerImagePlugin', 'LinkPlugin',],
        'limits': {'global': 1,},
    },
    'layout/other.html main': {
        'name': u'main content with FilerImagePlugin and no limit',
        'inherit':'layout/home.html main',
        'limits': {},
    },
}
```