xim: drop what no client uses; poll for owner loss like IBus

XIM text always went out through imdkit's COMPOUND_TEXT converter,
which already wraps every UTF-8 string in ESC%G; the UTF8_STRING
negotiation, the per-client encoding list, ximtext.c's fallback and its
wrapper never changed a byte on the wire. The spot location is honoured
for every style now (GTK's XIM module sends it with PreeditCallbacks)
and an unset focus window means the client window, as the spec says,
so those clients get the popup at the caret. readattrs/place kept four
transient fields to pass values between them; ximclose tore down state
right before die(); the OOM passthrough context was a third policy for
one small calloc where emalloc dies like everything else. Both frontends
now poll for engine-owner loss only while a preedit shows instead of
XIM waking on a pipe the engine had to know about; ibus stops waking
five times a second when idle. keymeaningful() is the engine's own
predicate. The standalone xim_test moved into the unit suite, so
xim/Makefile is gone.

xim/keymap.c

@@ -3,6 +3,7 @@
 #include <xkbcommon/xkbcommon.h>
 #include <xkbcommon/xkbcommon-names.h>
 
+/* The keysym for an X core key event, with its modifier and group state. */
 uint32_t
 keymaplookup(struct xkb_state *state, uint8_t keycode, uint16_t corestate)
 {
@@ -21,15 +22,13 @@ keymaplookup(struct xkb_state *state, uint8_t keycode, uint16_t corestate)
 	xkb_mod_mask_t mods;
 	int i;
 
-	if(state == NULL)
-		return XKB_KEY_NoSymbol;
 	keymap = xkb_state_get_keymap(state);
 	mods = 0;
 	for(i = 0; i < (int)(sizeof modname / sizeof modname[0]); i++){
 		if(!(corestate & (1U << i)))
 			continue;
 		index = xkb_keymap_mod_get_index(keymap, modname[i]);
-		if(index != XKB_MOD_INVALID && index < 8 * sizeof mods)
+		if(index != XKB_MOD_INVALID)
 			mods |= (xkb_mod_mask_t)1 << index;
 	}
 	xkb_state_update_mask(state, mods, 0, 0, 0, 0,