Hold Keys mode: keys stay down while touched, like a hardware keyboard

- HardwareKeys presses a key's unshifted keysym on touch and releases it
  on lift, through fake input; held keys repeat and combine as on a
  physical keyboard (games, Shift+click)
- Shift/Ctrl/Alt/Super are plain held keys, caps lock is the system's,
  dead keys compose in the app
- No long-press, flick, trackpad or rollover in this mode; nothing is
  left held when the mode is turned off or touches are cancelled
- Settings switch; default stays the touch (virtual keyboard) mode

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

tests/tst_keyboardsurface.cpp

@@ -135,6 +135,22 @@ private Q_SLOTS:
         QCOMPARE(engine.output, u"as");
     }
 
+    void holdKeysKeepsEveryFingerDown()
+    {
+        controller->setOptions({.holdKeys = true});
+        const QPoint w = keyCenter(u"w"_s), e = keyCenter(u"e"_s);
+        QTest::touchEvent(&window, touchscreen).press(0, w);
+        QTest::touchEvent(&window, touchscreen).stationary(0).press(1, e);
+        QTest::qWait(600); // past the long-press delay: e has accents, but no popup
+        QCOMPARE(engine.held, (QSet<quint32>{XKB_KEY_w, XKB_KEY_e}));
+        QTest::touchEvent(&window, touchscreen).release(0, w).stationary(1);
+        QCOMPARE(engine.held, QSet<quint32>{XKB_KEY_e});
+        QTest::touchEvent(&window, touchscreen).release(1, e);
+        QVERIFY(engine.held.isEmpty());
+        QVERIFY(engine.output.isEmpty()); // key events only, no text
+        controller->setOptions({});
+    }
+
     void slideFromNumbersKeyTypesAndReturns()
     {
         const QPoint layerKey = keyCenter(u".?123"_s, u"layer"_s);