Add (Un)Select Button

[shlake]

It would be good to have a "unselect all" button (and then of course a "select all" button) so users do not have to uncheck every file.

By default ALL file are checked, unchecking lots of files is tedious.

[qqmyers]

Seems like a useful feature. Note that it isn't ALL files that are checked, it is all files that aren't in the dataset already. So would the select/unselect avoid changing the state for ones already in the dataset? Or do we need a hide files already in Dataset option so that select/unselect all only works on the rest? Or?