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>

qml/Main.qml

@@ -9,6 +9,8 @@ Window {
     required property Settings settings
     property var placement: null // FloatingWindow, set from C++
     property bool settingsOpen: false
+    // Height of the key area, for C++ keeping key sizes when rows change.
+    readonly property real keysHeight: keyboard.height
 
     // Transparent room above the keyboard for long-press popups on the top row.
     readonly property real popupMargin: 90