Added inheritance option for configuring the available section pageparts

You can now extend from an other section pagepart configuration. 

Simple example:

```yml
#YourBundle/Resources/config/pageparts/default.yml
name: "Default"
context: "main"
types:
    - { name: "Header", class: "Kunstmaan\\PagePartBundle\\Entity\\HeaderPagePart" }
    - { name: "Text", class: "Kunstmaan\\PagePartBundle\\Entity\\TextPagePart" }
    - { name: "Line", class: "Kunstmaan\\PagePartBundle\\Entity\\LinePagePart" }

#YourBundle/Resources/config/pageparts/home.yml
name: "Home"
context: "main"
extends: "default"
types:
    - { name: "To Top", class: "Kunstmaan\\PagePartBundle\\Entity\\ToTopPagePart" }
    - { name: "Image", class: "Kunstmaan\\MediaPagePartBundle\\Entity\\ImagePagePart" }
```

Notes:
* It is possible to overwrite the types of the parent configuration file
* Multiple inheritance levels supported (eg. Default <-- Website <--- Home)
* Inheriting from multiple configurations possible (eg. `extends: ["default", "widgets"]`)