Add subseq operator to match sub-sequences

Summary:
Added subseq and ~subseq operators which match subsequences of strings.
This is useful to filter out matches quicker:

$ ktraderclient5 --constraint "'libremath' ~subseq Name" --servicetype 'Application' --short
servicetype is : Application
constraint is : 'libremath' ~subseq Name
got 1 offers.
/nix/store/zq1kgdxk426qgf06nzpd7xqkpk4z5hh2-system-path/share/applications/math.desktop

Test Plan: I'll need help knowing where tests for this will go.

Reviewers: dfaure, Frameworks

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13670