Similar to its `footer` content, ModalPopup can now receive header components
allowing consumers to design the header part of the modal to their likings.
Usage:
```
ModalPopup {
id: popup
header: Item {
...
}
...
}
```
The header section will take the height of the root element of the substree
passed to `header`
Pascal Precht
Jonathan Rainville
Richard Ramos