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/Keyboard.qml

@@ -8,6 +8,7 @@ KeyboardSurface {
         model: surface.model
         delegate: Key {
             shiftMode: surface.controller ? surface.controller.shiftMode : KeyboardController.Off
+            modifierModes: surface.controller ? surface.controller.modifierModes : ({})
             blank: surface.trackpadActive
         }
     }