Allow dictionary runner to search multiple times

Summary: Any time we search a word definition we add a source to our match engine but we never remove it.

Test Plan:
1) Open krunner
2) Search a word definition
3) Close krunner
4) Open krunner
5) Search the same word definition one more time
Expected result: the definition appears again
Actual result: there are no definitions shown.

Reviewers: jriddell, broulik, #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16038