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 height of file browser header
Use convenience methods to setup header/status bar backgrounds
Use system’s tool tip font for our custom tool tips
Tabs above document file browser header in tab bar

Integrate visually file manager header in tab bar when tabs above document
option in on. If tabs above document are off then use default gradient L&F.

Bottom divider for file browser header view is not integrated in header for
easier control if it is shown or not.