[Kickoff] Return Kickoff to Favorites page after running a search

Summary:
This patch fixes the bug that would make Kickoff retain the contents of the tab that was selected when a search was run from a tab other than
Favorites (but at the same time highlight Favorites in the Tab Bar, when it was in fact, stuck on another tab).

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18848