Drop search and routing results on planet change (QWidget tools only)

Summary:
A more complete solution could perhaps cache the results per planet.
So when switching back to a planet, the last search & routing results
could be used again.

Test Plan:
Switching between differet themes for the same planet keeps current
search & routing results. Switching planet drops the results.

Reviewers: rahn, nienhueser, #marble

Reviewed By: nienhueser, #marble

Differential Revision: https://phabricator.kde.org/D2087