Add QVector<Definition> Definition::includedDefinitions() const

Summary:
This function returns a list of all Definition that are reference
by IncludeRules. The returned list also contains the own Definition
as first entry.

Test Plan: make && make test

Reviewers: vkrause, cullmann

Reviewed By: vkrause

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D14468