BookmarksRunner: Avoid multiple connections of identical signal

Summary:
The factory returns the same object when the browser name is not changed.
Connecting the signal again leads to multiple calls to the slot each
time the signal is emitted.

See also T9626

Test Plan:

1. Add some debug output to the teardown() slot
2. Open the krunner multiple times and enter some query
3. teardown() is called exactly once

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: davidedmundson, broulik, plasma-devel

Tags: Plasma

Maniphest Tasks: T9626

Differential Revision: https://phabricator.kde.org/D15306