Requesting file previews should be limited in concurrency

[SystemKeeper]

Currently we request all previews for the files in a folder at once

nextcloud-dialogs/lib/components/FilePicker/FilePreview.vue

Lines 38 to 47 in 3e49a75

	watchEffect(() => {
	canLoadPreview.value = false
	if (previewURL.value) {
	const loader = new Image()
	loader.src = previewURL.value.href
	loader.onerror = () => loader.remove()
	loader.onload = () => { canLoadPreview.value = true; loader.remove() }
	}
	})

This will be problematic for folders with a large amount of files and/or previews that are not already cached. We should therefore limited the amount of requests we do in parallel as it is done on file deletion with a PQueue for example:

https://github.com/nextcloud/server/blob/025a7849b487351d0240d89833b3ab825897097d/apps/files/src/actions/deleteAction.ts#L106

Alternatively the previews should only be requested if they are close to "getting into the visible view"?!