Prevent users from adding the same remote file to a sample repeatedly

[ml-evs]

If I click "load file" when it is selected, 1) there is no feedback that the file has been loaded, and 2) it loads the file again every time I click load file.

We should:

a) change the UI after the file load request has been sent (e.g. close the file tree dialog, or somehow indicate that the file has been loaded already)
b) change the add-remote-file (or whatever it is called) to check for uniqueness in filename/path/file hash.