Improve and correct tool GUI strings

Summary:
Use correct spelling of "subtract"

Rename variables and elements to better show their purpose

Test Plan:
manual GUI testing

• inspect GUI element names
• read tooltips

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Subscribers: kalali, floca

Maniphest Tasks: T29349

Differential Revision: https://phabricator.mitk.org/D742