Fix for all themes: allow turn off attributes in XML highlighting files

Summary:
Depends on D23515

Use the new functions of KSyntaxHighlighting to verify if the bold, italic, underline & strikeout attributes are set in the XML files, so as to allow them to be turned off correctly in the themes that don't exist in KSyntaxHighlighting and that are applied by KTextEditor.

Reviewers: KTextEditor, cullmann, vkrause, dhaumann

Reviewed By: KTextEditor, cullmann, dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D23516