[KFind] Natural auto-scrolling during new items insertion into search results view

Summary:
When new results are added to the list, ensure KFind continues to display the results that user has scrolled to or selected.

Finally no more partial results constantly running away from cursor...

{F7048436}

Reviewers: #kfind, ngraham, davidedmundson

Reviewed By: ngraham, davidedmundson

Subscribers: broulik, davidedmundson

Differential Revision: https://phabricator.kde.org/D22547