Allow to change the Mode, after changing the Highlighting

Summary:
Currently, when selecting a language in the "Highlighting" menu, the "Mode" menu stops working.
This happens because, the Modes don't apply if m_hlSetByUser is true (if the user has chosen a Highlight in the menu).
This is swelling and I think it's better to change it.
Now when selecting a Mode, the "Highlighting" menu is also updated (but, when choosing an item in the Highlighting menu, the "Mode" menus don't change).

Reviewers: KTextEditor, Kate, cullmann

Reviewed By: KTextEditor, Kate, cullmann

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

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D20959