Full keyboard mode, Ctrl/Alt/Super, dead keys, shift levels

- Full (iPad Pro) mode: every typing key from the keymap, number row,
  tab, caps lock, ctrl/super/alt; shifted symbol shown above and flickable
- Dead keys compose with the locale's XKB compose table
- Ctrl/Alt/Super stick like shift and turn keys into shortcuts; a tapped
  or held shift joins shortcuts and non-text keys, auto-shift and caps
  lock don't
- Auto-capitalization and caps lock only capitalize letters; the user's
  shift picks the keymap's shifted level
- Mode switch in settings keeps key size by resizing the window

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

tests/tst_deadkeys.cpp

@@ -0,0 +1,43 @@
+#include "input/DeadKeys.h"
+
+#include <QTest>
+#include <xkbcommon/xkbcommon-keysyms.h>
+
+using namespace Qt::StringLiterals;
+
+class TestDeadKeys : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void composesWithTheNextLetter()
+    {
+        DeadKeys dead(u"en_US.UTF-8"_s);
+        QVERIFY(!dead.isPending());
+        dead.press(XKB_KEY_dead_acute);
+        QVERIFY(dead.isPending());
+        QCOMPARE(dead.compose(u"e"_s), u"é");
+        QVERIFY(!dead.isPending());
+
+        dead.press(XKB_KEY_dead_caron);
+        QCOMPARE(dead.compose(u"C"_s), u"Č");
+    }
+
+    void typesAccentAndLetterWhenNothingComposes()
+    {
+        DeadKeys dead(u"en_US.UTF-8"_s);
+        dead.press(XKB_KEY_dead_caron);
+        QCOMPARE(dead.compose(u"q"_s), u"ˇq");
+    }
+
+    void cancelForgetsTheAccent()
+    {
+        DeadKeys dead(u"en_US.UTF-8"_s);
+        dead.press(XKB_KEY_dead_acute);
+        dead.cancel();
+        QVERIFY(!dead.isPending());
+    }
+};
+
+QTEST_GUILESS_MAIN(TestDeadKeys)
+#include "tst_deadkeys.moc"