Selector for file transfers

[jl-wynen]

Given a file transfer like #38 and the already existing SSHFileTransfer, it would be good to have a way to say 'try using symlinks, if that doesn't work, use ssh'. This can probably be implemented as a wrapper, a file transfer class that has multiple children and selects an appropriate one. The file transfers themselves would then have to have a way for the parent to check if they can handle a given file.

See also the suggested file structure in #37