Add default variables for variables interface

Summary:
Add set of default variables that are similar to Qt Creator's variables.
These variables will be used in the external tools plugin as well as internally in KTextEditor e.g. to allow adding a date to backup files.

Test Plan: Added unit test + make && make test

Reviewers: cullmann, gregormi

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D21861