Tweak OakGradientView

We now call super instead of self with designated initializer since a subclass is likely to implement the designated initializer and from there call our “non-designated” one, causing an infinite loop, if we were to call back.

Also make class public.
Let file browser header inherit from OakGradientView
WIP: Add action bar to file browser
WIP: Use system controls in text view status bar
Let document view create status bar divider
Move file browser dividers into parent view.
Simplify constraints and set a height for the view