Move file browser divider into parent view
Tweak OakGradientView

- Accept two NSGradient objects instead of four colors.
- Call designated initializer on super instead of self since subclass is likely calling us from its designated initializer
- Make class public
Let file browser header inherit from OakGradientView
Simplify constraints and set a height for file browser header
Increase tab bar size one pixel

This is to match the minimum allowed size for the sidebar header when the tabs above document is enabled, also matches the height of the forthcoming tab redesign.
Shrink height when tabs above document is enabled
WIP: Add action bar to file browser
Restyle and add graphics to file browser action bar