Issues with clipboard permissions

There are two somewhat related issues with how clipboard permissions are handled:

• The new clipboard permission handling (see QTBUG-72714) unconditionally checks for JavascriptCanAccessClipboard. However, the documentation for that says: "Writing to the clipboard is always allowed if it is specifically requested by the user.". The same is true in Chromium, no special permissions need to be granted for e.g. a GitHub "copy link" button to work, but it doesn't work without setting JavascriptCanAccessClipboard with QtWebEngine.

• The same check checks for JavascriptCanPaste && JavascriptCanAccessClipboard which means both need to be set to true. In other words, it's impossible to allow a website write-only access to the clipboard, read access always needs to be granted as well.