Preserve color selection
Correctly initialize the color selection dialog
The QColorDialog can be initialized with the recently selected color.
The member variable color will only be changed if a valid color selection has been made inside the QColorDialog.
Test Plan: manual UI test
Reviewers: kahl, O1 MITK Reviewer Group I, kislinsk
Reviewed By: kahl, O1 MITK Reviewer Group I, kislinsk
Subscribers: kislinsk, kahl, kalali
Maniphest Tasks: T26969
Differential Revision: https://phabricator.mitk.org/D471