Fix StdMultiWidget initialization and add LevelWindowWidget
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