compose: one Compose table for the XIM and IBus frontends

An IBus client throws away a dead key its engine did not take — the GTK
module's own comment says so, and Qt does the same — so é and ü were
lost in every IBus application, while XIM composed them with a table of
its own.  That table moves to compose.c, which both frontends now use;
xim.c is the shorter for it.

A finished sequence is text, not a key: the engine is asked to hand back
what it had pending, and the composed text follows it, so the composed
character can no longer land before the syllable typed before it.

tests/compose_test.c

@@ -0,0 +1,37 @@
+#include "dat.h"
+#include "fn.h"
+#include "test.h"
+#include <xkbcommon/xkbcommon-keysyms.h>
+
+/* A sequence swallows its keys and hands its text back at the end. */
+void
+compose_sequences(struct ct *t)
+{
+	static const struct {
+		u32int	sym;
+		int	composing;
+		char	*text;
+	} keys[] = {
+		{ XKB_KEY_a,		0, "" },
+		{ XKB_KEY_dead_acute,	1, "" },
+		{ XKB_KEY_e,		0, "é" },
+		{ XKB_KEY_Multi_key,	1, "" },
+		{ XKB_KEY_o,		1, "" },
+		{ XKB_KEY_c,		0, "©" },
+		{ XKB_KEY_dead_acute,	1, "" },
+		{ XKB_KEY_Escape,	1, "" },
+		{ XKB_KEY_a,		0, "" },
+	};
+	char buf[Maxutf];
+	int i;
+
+	if(!CT_CHECK(t, setenv("XCOMPOSEFILE", "data/compose", 1) == 0))
+		return;
+	composeinit();
+	for(i = 0; i < nelem(keys); i++){
+		CT_EQ_INT(t, keys[i].composing,
+			composekey(keys[i].sym, buf, sizeof buf));
+		CT_EQ_STR(t, keys[i].text, buf);
+	}
+	unsetenv("XCOMPOSEFILE");
+}