Change widgets for new/rename label dialog

Summary:
Use a QLineEdit/QListWidget combination instead of a QComboBox.
It enables a streamlined selection of label name suggestions, i. e.
with a double click, which also accepts the selection and closes
the dialog.

Fixes T29288

Test Plan: Tested manually with short and long suggestion lists as well as in both dialog modes (new label vs. rename label).

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T29288

Differential Revision: https://phabricator.mitk.org/D710