[upload] Add maxFilesAtOnce property

[davidmaxwaterman]

On some connections, uploading multiple files can be problematic. I think it would be useful to have a maxFilesAtOnce option that the user would be able to configure it to minimise concurrent connections.

[davidmaxwaterman]

ok, I was able to implement this in vaadin-upload's parent element reasonably tidily using the various events.

I maintained two arrays, uploadsInProgress and queuedRequests.

In the upload-request handler, if the uploadsInProgress.length > maxFilesAtOnce, I preventDefault and add the event.detail to queuedRequests; otherwise I add it to uploadsInProgress.

In the handlers for upload-error, upload-abort, and upload-success, I find the index of the event.file in uploadsInProgress and remove it; and if the queuedRequests.length>0, pop off the next one, add it to uploadsInProgress, and call xhr.send() on it (with the next.file.fileBuffer as a param, in my case[0]).

Anyway, I thought I'd let you know my working algorithm, in case you want to at least make an example of how to do it, even if you don't implement a 'maxFilesAtOnce' property.

[0] because I've hacked my fork of vaadin-upload to use a fileBuffer from FileReader.