dat: one pair of content purposes

Two frontends read the same two numbers, and wl.c carried four lines of
comment to say why it kept its own copy of them.  IBus and text-input-v3
number the purposes alike because both took them from GTK, so the header
is where they belong and the explanation goes away with the copy.  hidden
stays in both files: one asks about a context, the other about the seat.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ibus.c

@@ -16,8 +16,6 @@ enum
 	Relmask		= 1<<30,
 	/* IBus wire constants; the daemon does not link libibus. */
 	Ibuscappreedit	= 1<<0,
-	Ibuspurposepassword = 8,
-	Ibuspurposepin	= 9,
 	Ibusattrunderline = 1,
 	Ibusunderlinesingle = 1,
 	Ibuspreeditclear = 0,
@@ -313,8 +311,8 @@ sendrequest(Ictx *ctx, int op, u32int ks, u32int mod, Keyres *res)
 static int
 hidden(Ictx *ctx)
 {
-	return ctx->purpose == Ibuspurposepassword ||
-		ctx->purpose == Ibuspurposepin;
+	return ctx->purpose == Purposepassword ||
+		ctx->purpose == Purposepin;
 }
 
 static void