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/cmdhelp

@@ -82,6 +82,7 @@ Z       Zap (cast) a spell
 &       Tell what a command does
 !       Do a shell escape (only if defined)
 \       Show what object types have been discovered
+`       Show discovered types for one class of objects
 _       Travel via a shortest-path algorithm to a point on the map 
 .       Rest one move while doing nothing
         Rest one move while doing nothing (if rest_on_space option is on)