Select all and deselect all buttons at datafile level

[tg137]

One request has been to add select/deselect all byttons at the datafile level.

A comment has been that doing this essentially just adds an entire dataset but it should be treated differently to this such that users can pick and choose without their files being locked down.

It should also be configurable because facilities with huge numbers of datafiles in their datasets would not want to iterate over them all.