Add placeholders for the search lines

Summary:
Some people need them

BUG: 403445

Test Plan: Can be compiled, installed, can be spotted in the GUI.

Reviewers: Okular, ngraham

Reviewed By: ngraham

Subscribers: ngraham, VDG, GB_2, okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D18429