Create new VirtualFileCollection model

[prioux]

This new model would represent virtually a set of other CBRAIN file as a FileCollection of symlinks; synchronizing it would synchronize all the CBRAIN files and create symlinks to them in the top of the FileCollection. The list of CBRAIN files would be maintained in a hidden CbrainFileList-compatible file.

We already have something very similar to this in https://github.com/aces/cbrain-plugins-neuro in the model called CivetVirtualStudy .

Note that several important synchronization-related overrides need to be added the the model's behavior (see CivetVirtualStudy for examples).

[MontrealSergiy]

1. Following above requirements, the VirtualFileCollection works well (I created a fork for it in my profile). Since links do not support well merging of similarly named folders or files, a check is added. However it is not clear what should happen if one of sub-elements is renamed? One can either silently ignore the issue, send a warning or block rename, or perhaps somehow keep the old name?

2. To avoid cyclic situations multilayer VirutalFileCollection are disallowed.