QmlDesigner.DesignDocument: simplifying the TextModifers

The TextModifers for the sub component and the document
are now assigned to the model. This simplfifies the code.

The member variables to the TextModifers are only kept for ownership.
The DesignDocument still owns the two TextModifers.

Change-Id: Idc84a2ba718666ce54683a67635a93352784dddd
Reviewed-by: Marco Bubke <marco.bubke@digia.com>