[TermGenerator] Use UTF-8 ByteArray for termList

Summary:
All users of TermGenerator::termList do not use QStrings but UTF-8
ByteArrays. Remove repetitive code.

While at it, make the lists const to avoid detach on iteration.

Test Plan: ctest

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21839