Load images asynchronously in media details view
Loading images takes often very long if full size images with e.g. ~10MB are involved. Currently the TaxEditor becomes unresponsive during load which makes editing images very cumbersome. We need to load images asynchronously (same as in dataportal).
This also holds for the image gallery view if "show thumbnails" is switched on (but hopefully there only thumbnail images are loaded if available - I did not check).