Source fill label value from active layer
Summary: Fixes T29368
Test Plan: Use the fill tool in different layers. You can use F2578705 to fill the green circle.
Reviewers: O1 MITK Reviewer Group I, floca
Reviewed By: O1 MITK Reviewer Group I, floca
Subscribers: floca
Maniphest Tasks: T29368
Differential Revision: https://phabricator.mitk.org/D748