Add a "select all" option for batch slicing

[butlerpd]

as commented by @katieweigandt in PR #1721 the batch slicer works very well until one wants to apply a batch slicer to 60 files for example. Having to click on each one individually is quite daunting. This could be coupled with with issue #1181 replacing the list of 2D plots with the list of 2D data in the data manager for example.