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/CMakeLists.txt

@@ -6,6 +6,8 @@ qt_add_library(sloposk_core STATIC
     engine/InputEngine.h
     engine/DirectEngine.h engine/DirectEngine.cpp
     input/ShiftState.h input/ShiftState.cpp
+    input/Modifiers.h input/Modifiers.cpp
+    input/DeadKeys.h input/DeadKeys.cpp
     input/PointerTracker.h input/PointerTracker.cpp
     input/AlternatesPicker.h input/AlternatesPicker.cpp
     input/KeyRepeater.h input/KeyRepeater.cpp
@@ -15,7 +17,7 @@ qt_add_library(sloposk_core STATIC
     input/KeyboardSurface.h input/KeyboardSurface.cpp
     layout/Layout.h
     layout/LayoutLoader.h layout/LayoutLoader.cpp
-    layout/KeymapLetters.h layout/KeymapLetters.cpp
+    layout/KeymapKeys.h layout/KeymapKeys.cpp
     layout/LayoutBuilder.h layout/LayoutBuilder.cpp
     layout/KeyGeometry.h layout/KeyGeometry.cpp
     layout/LayoutModel.h layout/LayoutModel.cpp