ShowManager: fix show update information when image exists
So far updating the show always worked out nicely, as long as no logo/image has been set. But as soon as a logo or image is set, we cannot just insert the whole show object into the PUT request body, because if an image is submitted we have to use multipart/form-data and we do not want to always reupload images.
Therefore every modal that updates one info of a show should fetch a new updatedShow object with the minimal set of infos that are required for a PUT to the show endpoint, and then add/change the specific info that should get updated.