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.

src/artifact.c

@@ -835,6 +835,8 @@ winid tmpwin;		/* supplied by dodiscover() */
 
     for (i = 0; i < NROFARTIFACTS; i++) {
 	if (artidisco[i] == 0) break;	/* empty slot implies end of list */
+	if (tmpwin == WIN_ERR) continue; /* for WIN_ERR, we just count */
+
 	if (i == 0) putstr(tmpwin, iflags.menu_headings, "Artifacts");
 	m = artidisco[i];
 	otyp = artilist[m].otyp;