Improve the user experience for selecting directories when using the File System driver

[jrgriffiniii]

It has been found by users (when this is integrated into the Valkyrie-based Samvera repository implementation Figgy) that the directory icon is not intuitive to users who are seeking to expand the contents of a directory in the tree browser:

There should be a separate icon, positioned to the left of the directory, used to expand the tree of files.