Remove FileList return from br::Enroll

With this, we will not have to accumulate the file metadata during enroll
which will help with memory use on large jobs.

Functions actually using the return type (gui/classifier.cpp, and gallerytoolbar.cpp)
updated to avoid using it directly. In gallerytoolbar.cpp, drop support for
implicit enrollment to a .gal file.
Explicitly surface the behavior of the end of a stream as a transform

By default, all processed templates are collected, and output at the end
of the call to projectUpdate, but it is now possible to e.g. simply discard all
results, if nothing more needs to be done with the processed templates.
Merge pull request #223 from biometrics/enroll_update

Drop filelist return from enroll, don't accumulate metadata during enroll