Optional PC key blocks: function row, navigation, numeric keypad

- Template sections placed above or to the right of the typing keys;
  the user picks any mix in settings, typing keys included
- keysym keys send XKB keysyms on press, optionally repeating
- Geometry lays blocks side by side with one key unit; stretch keys are
  resolved at layout time so composed blocks line up
- Toggling a block resizes the window so keys keep their size

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

src/CMakeLists.txt

@@ -32,6 +32,7 @@ set(SLOPOSK_QML_FILES
     AlternatesPopup.qml
     DragHandle.qml
     ResizeHandle.qml
+    SectionHeader.qml
     SettingsButton.qml
     SettingsPanel.qml
     ToggleRow.qml