Variable expansion: Add variable PercentEncoded

Summary:
This can be used to encode the input text such that it can
be used in URIs. This is useful e.g. for the external tool
"Google Selected Text", where '&' and similar characters
are problematic in the url.

CCBUG: 416509

Test Plan: make && make test

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D26812