feat: multiple file upload (#499)

[tomicvladan]

Multiple file upload.

This can cause some unexpected situations. When multiple files are uploaded, if only one of them fails, the error will be displayed in the modal. Other files might be uploaded successfully but the user won't be aware. Also because of the error current folder content won't be refreshed.

[IgorShadurin]

It might be worth considering disabling multiple uploading. Since at the moment there is no optimal implementation for uploading a custom list of files on the fdp-storage side.
Now we can optimally upload only the entire directory with files using fdp.directory.upload. In future releases I may do something similar for the list of files.

Now loading the file one by one will not be very fast + errors are possible on the bee side due to non-instant access to chunk data.

[tomicvladan]

I agree, that implementation is not usable. But now I updated the branch with new functionality. When multiple files are selected, they are all listed in the upload modal. When one file either uploads or fails it will change its color, indicating its status. All files are uploaded sequentially. When all files are uploaded successfully then the modal closes as before. If some of the files fails, then its name is displayed in red color and the modal stays open. User can try again by clicking the upload button, and only the failed ones will be reuploaded. If the user chooses other files, the whole state resets.