Add preferences for existing UI style controls

Some puzzles have keys that make changes to the display style in ways
that would probably have been user preferences if they had existed.
I've added a user preference for each of these.  The keys still work,
and unlike the preferences can be changed without saving any state.

The affected settings are:
 * Labelling colours with numbers in Guess ("L" key)
 * Labelling regions with numbers in Map ("L" key)
 * Whether monsters are shown as letters or pictures in Undead ("A" key)

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