Any masterpage editing tool requires some server-side (or desktop) implementation in order to apply and save changes to files on the web server. In addition, the discussed functionality is not a UI component. These are the two main reasons why such a feature is out of the scope of Kendo UI and offering it as part of the framework is unlikely. Thank you for you feedback, though.
I can tell you how we are currently handling the Kendo UI demos - we have HTML snippets for the reusable HTML code that act like a masterpage. We "compile" the reusable snippets and the demo-specific HTML with Node.js. You can consider using a similar approach (if possible) in order to avoid multiple file edits.
the Telerik team
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