Fix blank help pages
Summary:
Let berry::HelpPluginActivator manage the URL scheme handler for qthelp.
This fixes blank help pages as soon as the help editor is opened for the second time.
Fixes T30413
Test Plan:
- Open a help page
- Close the help editor
- Open another help page
- Check if the help page is not blank
- Close the Workbench and check if it closed gracefully with return code 0
Reviewers: O1 MITK Reviewer Group I!
Maniphest Tasks: T30413
Differential Revision: https://phabricator.mitk.org/D948