Initialize encoding pop-up (Preferences)

Previously this would always show UTF-8 and cause an existing setting to be overwritten (with UTF-8) when bringing up the preferences window.
Use ‘nil’ to indicate no represented file

The window uses the empty string, but since we use our represented file property to create proxy icons, test if we should setup SCM status watching, etc., we don’t want to test for either nil or empty string in all these places.

This fixes exception from creating OakFileIconImage from empty string.
Restyle navigation bar to be more in line with Lion.

- The shading is a combination of the scope bar and textured button used in Lion/Mountain Lion.
- Add an inactive state.
- Increase click area of the forward/back buttons while decreasing the image size.
- Switch popup menu cell to NSBackgroundStyleLight, previously the text would go to gray when inactive this prevents that though it does lose the drop shadow.
- Various tweaks to alignment and spacing.