Support CopyAsImage in iframe

[YuxinZhaozyx]

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Is your feature request related to a problem? Please describe.
I embed the draw.io editor in an iframe, but "Copy As Image" is not shown in the right-click menu.

Describe the solution you'd like
I have checked the source code. The menu item "Copy As Image" is set to invisible when Editor.enableNativeClipboard is false, while Editor.enableNativeClipboard is set to false in iframe (Editor.js):

/**
 * Specifies if the native clipboard is enabled. Blocked in iframes for possible sandbox attribute.
 * LATER: Check if actually blocked.
 */
Editor.enableNativeClipboard = window == window.top && navigator.clipboard != null &&
	navigator.clipboard.read != null;

The comments of this line indicate that the native clipboard should not be blocked in iframe if sandbox attribute is unset.

[alderg]

We will add a config switch called enableNativeClipboard in the next release.

[mwiedemeyer]

The issue here is window==window.top.
My workaround for https://diagramHub.app was to patch this in app.min.js and replace it with an empty string and also set this iframe attribute: <iframe allow="clipboard-read; clipboard-write" (...)
The new flag would be great