Workspace preview does not transfer query parameters
If the URL called up when previewing from the TYPO3 backend contains additional query parameters, these should be passed on to the URLs of the iframe panels of the preview view.
I have no scenario from the Core to repeat this, but I was in a situation where I manually added query parameters to the URL and was surprised to find them being ignored. This is why I'm not submitting this patch outright, but rather just uploading it here for discussion.
Incidentally this patch resolves a "todo" that was mentioned inside the PreviewController about the language parameter not always being "L". With this patch all non-workspaces related GET query parameters are retrieved, so there's no worrying about which should be the language parameter.