improve responsiveness when select folders that have subfolders #2
Description
As the content of a folder can only be known when its containing web page is open, the extension has to recursively load all subfolders of a folder to download all files in it. But when a user is intended to download a whole folder and don't care what its content is (surely he knows but just don't want to deselect anyone), this loading action causes a stall of user interaction. Except selecting which file to download, the whole folder structure is only useful to display file counts of each file type, which are read-only values and cannot be changed as long as the user still want to download the same files, so that are only assisting statistics and should not cause the user to stop his action, neither. So, we can delay the fetching of a folder's content to when we must use it, that is when the user expands that folder to see its content and when we start downloading. The file filter can be applied just before the downloading. For the purpose of file count, it may be displayed after fetching the structure and before downloading as a confirmation action, or just add a "review download list" action or similar.