Summary:
Previous approach was oversimplified and did not account all possible cases (autogenerated doc lists and glossaries).

BUG: 417222

Test Plan:
1. Open KHelpCenter.
2. Double click on "Application manuals" in the sidebar.
3. Click on "Increase font sizes" or "Decrease font sizes" button in toolbar.
4. Font should be changed as expected.
5. Open some manual, repeat 3.
6. Open some glossary entry, repeat 3.
7. Font size change should be fine.

Reviewers: #kde_applications

Subscribers: kde-doc-english

Tags: #documentation

Differential Revision: https://phabricator.kde.org/D27196