Dead keys: act on compose status at press time

- Two dead keys that compose (´´ → ´) type the result right away; a dead
  key that can't continue the sequence types the old accents and starts anew
- With Ctrl/Alt/Super, a dead key sends its dead keysym, as the physical
  key does, not the spacing accent from its label

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/tst_deadkeys.cpp

@@ -30,6 +30,16 @@ private Q_SLOTS:
         QCOMPARE(dead.compose(u"q"_s), u"ˇq");
     }
 
+    void twoDeadKeysComposeAtOnce()
+    {
+        DeadKeys dead(u"en_US.UTF-8"_s);
+        QVERIFY(dead.press(XKB_KEY_dead_acute).isEmpty());
+        QCOMPARE(dead.press(XKB_KEY_dead_acute), u"´");
+        QVERIFY(!dead.isPending());
+        dead.press(XKB_KEY_dead_acute); // a fresh sequence
+        QCOMPARE(dead.compose(u"e"_s), u"é");
+    }
+
     void cancelForgetsTheAccent()
     {
         DeadKeys dead(u"en_US.UTF-8"_s);