Enforce use of the reset timer for clearing the model

Summary:
Currently when the query string changes, krunner sends a signal with no matches
directly, which circumvents use of the reset timer.

Test Plan:
Enter =1+1, results are shown. Add another "1", the shown 2 switches to a 12
instantly without hiding the previous result for a few frames.

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21670