Use KSyntaxHighlighting::Theme infrastructure for color themes in KTextEditor
Instead of relying on KConfig files we ship, just use the theme data from KSyntaxHighlighting.
Use the theme data as fallback for reading from KConfig stuff. If we do it that way, the editing parts of the colors/... can stay as it is, we just need to ensure we use the correct data as default values.
Perhaps Dominik can take a glance at this, if it makes sense at all.