Accessibility: Fix message boxes not being announced

This commit moves the focus to the message of newly shown passive messages when accessibility software is used. Moving focus to an object generally means that it will be announced, so this makes sure that users of accessibility software know when they are being notified of a change.

Needs frameworks/kwidgetsaddons!270 (merged) and an enabled screen reader to experience the intended user experience for announcing error messages. Navigate e.g. to a folder you don't have access to view to let an inline notification appear.