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