Programmatically setting FileSelectors is inflexible

[jlstevens]

This has been a problem for me on several instances - I try to set my own path on FileSelector but find I am not allowed to due to the path glob. For instance, I tried to use JSONInit to set the path to a file but then I get TraitError: Invalid selection even though I want to select my own file (i.e not in the glob list).

For this reason, I have had to resort to using a param.String for now...