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