Refactoring document handling

The document handling in the qml designer was complicated source
code, which was initially intended for a non creator application.
To integrate new views it has to be changed and cleaned up. This
is the first major step in that direction.

Change-Id: Ie26f0aad7a03946d18bdb4c0759b246c5439d922
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>