Rework Dash Preview column margins
If there's one column
Preview widgets have 2GU on each side
Unless it's one of the "full screen" widgets, i.e. video/image
This hasn't changed in this patch
If there's two columns
the columns have 4GU on each side and between eachother
This has changed according to design guidelines
Approved by: Andrea Cimitan, Unity8 CI Bot