Fix missing open / modified status in file chooser

This only worked for untitled files since we no longer keep document UUIDs for documents with a path.
Use divider lines in root view

Previously we placed the views with a one point gap between them and had the superview fill the background. This works fine, but has the somewhat theoretical disadvantage that the superview then needs to implement drawRect: and for performance reasons declare that it is “opaque” which means subviews can’t use the (true) window background, should they want to.

Also add a preferences key for “tabs above document” (issue issue #214), but not hooked up in this commit.
Tab bar above document option

This change adds new option for showing tab bar only above document, so the
file browser header lines up with tab bar.

File browser header height reduced by 1 pixel to match tab bar height. It also
draws optional top divider when in same line as tab bar
Initial divider colors were wrong for inactive windows