Don't delay emission of matchesChanged indefinitely

Summary:
Currently the signal is only emitted if there was no change to the matches in
the last 100ms. Especially during krunner startup and early result collection,
this is unlikely to happen though, so make sure that the signal is emitted
at least once every ~500ms.

Test Plan:
Sometimes results only appeared after a noticable delay, now this
delay is much shorter.

Reviewers: Frameworks, broulik, ngraham

Reviewed By: ngraham

Subscribers: ngraham, bruns, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21607