Fix StdMultiWidget initialization and add LevelWindowWidget

Summary:
The QmitkStdMultiWidget was changed by another feature to be an QmitkAbstractMultiWidget, which requires explicit render window initialization.
Additionally the LevelWindowWidget is not controlled by the multi widget anymore. Step8 adds the LevelWindowWidget explicitly.

Test Plan: manual

Reviewers: thomass, kislinsk

Reviewed By: thomass, kislinsk

Differential Revision: https://phabricator.mitk.org/D320