simplify preference editing by using a split view for default and user files

Editing files to set preferences is nice and flexible, but it's a bit of a pain to have to select the default file first (to see what's available) and then select the user file (to make changes). That also subtly encourages editing that default file. Here's a simple solution:

- For each kind of preference in the Preferences menu, display a single item: the name of the preference.

- Selecting that item displays both the default file and the user file in a split view, e.g. defaults above, user below. The user can then read the desired info from the top pane and copy the appropriate lines into the other.

- The Preferences menu item could then be much simpler: a single entry for each preference would display the pair of files (as just described).

To reiterate, I see these advantages:

- Simpler Preferences menu: one item per kind of preference.

- It is easier to for the user to edit preferences: one selection does it all.

- There is less temptation to edit the default file.