Assistant: Fix opening links as new pages

Pressing MMB or Ctrl+LMB opens now the link as a new page.
Clicking RMB shows now the context menu with opening page options.

Fixes: QTBUG-130137
Fixes: QTBUG-109158
Pick-to: 6.12
Change-Id: I66d3fde8e7957cbdc48c1d1394eb0e0f6d47d592
Reviewed-by: Kai Köhne <kai.koehne@qt.io>