ContextChecker: support '!' context switchting and fallthroughContext

Summary:
Extend the highlighting index checker to support contexts of
the form "#pop!otherContext". Additionally, scan for fallthroughContext,
if applicable.

Test Plan: make && make test

Reviewers: cullmann, vkrause

Reviewed By: vkrause

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7951