Validate that for all attributes an itemData exists

Summary:
This patch adds a check at compile time that verifies that for
all refereced attributes an itemData item must exist. If not,
a failure is raised, stopping the process of creating the Qt
resource file that contains the syntax highlighting files.

Currently, only warnings are raised (no compile time error),
since there are still some remaining issues that need to be
fixed before we can change to a compile-time error.

Test Plan: make && make test && make install

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9125