Tablet: Grid settings (number of grid columns)

[nicklepine]

We recently introduced #1352 for letting buyers control the number of columns in the grid sections on mobile and tablets. If and when the theme editor introduces the ability to edit the theme in a tablet viewport, we would like to eventually introduce a tablet specific setting for "number of columns". Defaulting to the mobile-specific settings causes for the 1-column or 2 column grids to be quite wide, so offering settings specific to table would help.

[MagnusHJensen]

This could be a very nice feature, currently have to do this manually since the product images for featured collections on screen just below 990 are WAYYY to big.

And it feels like a hassle to do, trying to get 1 col on mobile, 2 on tablet and 3 on desktop.