Highlighting Indexer: Warn about duplicate contexts

Summary:
Warn, if two contexts have the same name.

Currently, we have the following issues:
ContextChecker::processElement: "data/syntax/cgis.xml" context duplicate: "Region Marker"
ContextChecker::processElement: "data/syntax/cgis.xml" context duplicate: "Commentar 1"
ContextChecker::processElement: "data/syntax/cgis.xml" context duplicate: "Commentar 2"
ContextChecker::processElement: "data/syntax/diff.xml" context duplicate: "File"
ContextChecker::processElement: "data/syntax/objectivecpp.xml" context duplicate: "Preprocessor"
ContextChecker::processElement: "data/syntax/spice.xml" context duplicate: "String"

Test Plan: make && make test

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9132