add chunking for FilesAdded to improve responsiveness when scanning dropped folders for 1000s of files

add IMAGE_EXIF_MISSING_ERROR, check for valid JPG Exif meta tags before upload
add plupload.SKIPPED error status, skip uploading file, typically used for files with missing Exif data
add settings.use_lazy_preload, to preload img on when scrolled into thumbs view
debug changes