web interface: capture action needs feedback about spreads

[jbaiter]

👍
My idea was to overlay the application with a clearly visible spinner that signals that capture is in progress and prevents the user from clicking any buttons.
Also I want to implement a small counter for how many images have been shot already and eventual warnings/error messages.
And long polling does not have anything to do with that :-)

The process is currently as follows:

1. The client fires a POST request to /workflow/:id/capture
2. The server triggers the capture on the devices, waits for completion
3. The server answers with a JSON object consisting of the addresses of the images, some statistics (currently only number of pages shot, though this will include error/warning messages as well)
4. The client updates its workflow model with the new information and then the UI

That is, the request is only completed when the capture has completed and there's no need for polling to wait for an update.

Long polling is currently only needed when two users view the webinterface at the same time, the client not involved in the capture will see all updates the other user makes without having to refresh his browser.
This will become more useful when I implement the postprocessing side, then the interface can give direct feedback on how far in it is without the user having to do anything.
This is different from the capture process, since here it is the client who clearly initiates the changes, so he can be notified directly, via the server's answer to his POST request.

Pietr is very welcome to work on the webinterface, he will have to get acquainted with Foundation and React.js, though, since these handle the view component of the client application. And some knowledge on how to use Backbone models wouldn't hurt either :-)

[markvdb]

This is running now. Great, thx Johannes!