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>

src/input/DeadKeys.cpp

@@ -0,0 +1,68 @@
+#include "DeadKeys.h"
+
+#include "layout/KeymapKeys.h"
+
+#include <xkbcommon/xkbcommon-compose.h>
+#include <xkbcommon/xkbcommon.h>
+
+namespace {
+
+QByteArray systemLocale()
+{
+    for (const char *name : {"LC_ALL", "LC_CTYPE", "LANG"}) {
+        if (const QByteArray value = qgetenv(name); !value.isEmpty())
+            return value;
+    }
+    return "C";
+}
+
+} // namespace
+
+DeadKeys::DeadKeys(const QString &locale)
+    : m_context(xkb_context_new(XKB_CONTEXT_NO_FLAGS), xkb_context_unref)
+    , m_table(nullptr, xkb_compose_table_unref)
+    , m_state(nullptr, xkb_compose_state_unref)
+{
+    // The C locale has no compose table; en_US.UTF-8's is the common base of all.
+    const QByteArray name = locale.isEmpty() ? systemLocale() : locale.toUtf8();
+    for (const QByteArray &candidate : {name, QByteArrayLiteral("en_US.UTF-8")}) {
+        m_table.reset(xkb_compose_table_new_from_locale(m_context.get(), candidate.constData(),
+                                                        XKB_COMPOSE_COMPILE_NO_FLAGS));
+        if (m_table)
+            break;
+    }
+    if (m_table)
+        m_state.reset(xkb_compose_state_new(m_table.get(), XKB_COMPOSE_STATE_NO_FLAGS));
+}
+
+DeadKeys::~DeadKeys() = default;
+
+void DeadKeys::press(quint32 deadKeysym)
+{
+    m_pending += deadKeyText(deadKeysym);
+    if (m_state)
+        xkb_compose_state_feed(m_state.get(), deadKeysym);
+}
+
+QString DeadKeys::compose(const QString &text)
+{
+    QString result = m_pending + text;
+    if (m_state && !text.isEmpty()) {
+        const char32_t ucs = text.toUcs4().first();
+        xkb_compose_state_feed(m_state.get(), xkb_utf32_to_keysym(ucs));
+        if (xkb_compose_state_get_status(m_state.get()) == XKB_COMPOSE_COMPOSED) {
+            char utf8[64];
+            xkb_compose_state_get_utf8(m_state.get(), utf8, sizeof utf8);
+            result = QString::fromUtf8(utf8) + text.mid(QString::fromUcs4(&ucs, 1).size());
+        }
+    }
+    cancel();
+    return result;
+}
+
+void DeadKeys::cancel()
+{
+    m_pending.clear();
+    if (m_state)
+        xkb_compose_state_reset(m_state.get());
+}