Move from the searchbox to the results with the down arrow key

Summary:
Move from the searchbox to the search results listbox/view using the
down arrow key in addition to the existing methods using the tab key,
return key, or the mouse.

Test Plan:
use ctrl+f to search in a directory tree, press down arrow key
to go to the results

Reviewers: ngraham, Dolphin

Reviewed By: ngraham, Dolphin

Subscribers: meven, elvisangelaccio, ngraham, iasensio, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D26362