Add NSGradient support to OakBackgroundFillView
Use OakBackgroundFillView instead of OakGradientView
Remove OakGradientView
Make file browser’s header view a public property

This allows users of the file browser to setup constraints involving the file browser header or change its (background) styling.
Don’t allow tab bar view to expand beyond its intrinsic height

This makes it easier to constrain other views to the height of the tab bar view, without the tab bar view growing to the height of the other view.
Let ProjectLayoutView know about the file browser header view
Let ProjectLayoutView setup file browser header’s height