Re-implement launching task arrays by selecting multiple files in New Boutiques

[prioux]

The old boutiques integrator allowed a user to launch a set of tasks by selecting a bunch of files and leaving one file input blank; multiple tasks were launched and each of them got one file in that input.

The new boutiques integrator forces the user to create a CbrainFileList. I did it like that because I was re writing the launch code form scratch, but I think getting the old behavior to work too would be useful.

If a task has several file inputs, only one can be left blank, of course.

Make sure the 'preview' button's code says the right thing too.

[natacha-beck]

Closed by #1298