Assistant: Fix passing focus to help viewer after closing find bar

Before we were setting the focus to HelpViewer, but it wasn't
passed to its child QLiteHtmlWidget.
Setting focus proxy HelpViewer -> QLiteHtmlWidget also
doesn't work correctly.
Add explicit HelpViewer::doSetFocus() as a workaround.

Fixes: QTBUG-101045
Pick-to: 6.12
Change-Id: Ie1d55e8968f3306ace9e6b47e8614de38bbf936c
Reviewed-by: Kai Köhne <kai.koehne@qt.io>