make krunner accessible

Summary:
enable actual keyboard navigation, as the previous
completely manual keyboard navigation in krunner completely broke
the screen reader support, rendering it inaccessible (while
potentially krunner is one of the most useful apps for blind
users)

Test Plan:
tested to navigate with keyboard (still similar behavior as before)
while having orca running, which correctly read the result entries

Reviewers: Plasma, davidedmundson, broulik

Reviewed By: Plasma, davidedmundson

Subscribers: gladhorn, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D7765