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/test.h

@@ -112,6 +112,7 @@ void	ibus_capability_policy(struct ct*);
 void	ibus_private_input_policy(struct ct*);
 void	ibus_context_lifecycle(struct ct*);
 void	ibus_active_release_lifecycle(struct ct*);
+void	compose_sequences(struct ct*);
 void	xim_keymap_lookup(struct ct*);
 void	xim_compound_text(struct ct*);
 void	xim_adapter_key_contract(struct ct*);