T29548 Optimize_QmitkMultiLabelManager_buttons
Summary:
Reworked button icons.
Added DeleteLabelInstance btn to QmitkMultiLabelManager
Fixed crash caused by quering model in invalid state when updating controls
+ optimized UI
+ fixed T29548
Test Plan: code review + application testing
Reviewers: O1 MITK Reviewer Group I, kislinsk
Reviewed By: O1 MITK Reviewer Group I, kislinsk
Subscribers: kislinsk
Maniphest Tasks: T29548
Differential Revision: https://phabricator.mitk.org/D817
