Show if KRunner is still searching for more things

Summary:
Shows a busy indicator in the TextField.

Depends D22723.

BUG: 409959

Test Plan: Searched things, eventually it disappears, I sometimes don't really know what's still doing, maybe we could add some more information at some point.

Reviewers: Plasma, VDG, broulik

Reviewed By: Plasma, VDG, broulik

Subscribers: mart, davidedmundson, ngraham, aspotashev, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22514