Consider implement copy and paste function with LocalStorage on systems which do not natively allow copy & paste

[acane77]

When use it in browser on iOS, select an area and try to copy, a messages says “method not allowed”, although the system may do not allow web pages write to clipboard or read from clipboard, an in-app copy & paste strategy can be considered, which just need a place (e.g., in LocalStorage) to store copied images and images can be pasted to the same web page, it will be beneficial when drawing on mobile devices.