QmlDesigner.DesignDocument: fixes delete and cut

For deleting Items we need a QmlModelView to delete
dangling property changes.
In the past we "missused" the FormEditorView.
For now VewManager will expose the form editor as
a QmlModelView.

I also cleaned some includes and removed debugs.

Task-number: QTCREATORBUG-8650
Change-Id: I22ca7c5f741324532e81918571be39babd12d2e4
Reviewed-by: Marco Bubke <marco.bubke@digia.com>