Fix hidden languages in the mode menu

Summary:
In the status bar, the syntax highlighting menu (mode menu) shows hidden languages, for example:

Others -> Alerts
Others -> Modelines

Reviewers: KTextEditor, cullmann

Reviewed By: KTextEditor, cullmann

Subscribers: cullmann, dhaumann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D17083