Preserve color selection

Summary:
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