new command '`' to show discoveries for one class (trunk only)

Use the grave accent (back tick) character as the keystroke for a
new command which prompts for an object class and then shows a subset of
the discovered objects list covering just the selected class.  Similar
to the 'I' variant of 'i' for viewing inventory, and mainly useful once
the '\' discoveries list has grown long.

dat/help

@@ -166,6 +166,7 @@ Commands:
         $       Count your gold pieces.
         +       List the spells you know; also rearrange them if desired.
         \       Show what types of objects have been discovered.
+        `       Show discovered types for one class of objects.
         !       Escape to a shell, if supported in your version and OS.
         #       Introduces one of the "extended" commands.  To get a list of
                 the commands you can use with "#" type "#?".  The extended