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>

tests/tst_keygeometry.cpp

@@ -49,6 +49,26 @@ private Q_SLOTS:
         QCOMPARE(rects[4].left(), 100.0);
         QCOMPARE(rects[5].right(), 300.0);
     }
+
+    void stretchKeysFillTheRow()
+    {
+        KeyDef wide = key();
+        wide.stretch = true;
+        const Layer layer{u"l"_s, {{key(), key(), key(), key()}, {key(), wide}}};
+        const auto rects = computeKeyRects(layer, {400, 100}, {.padding = 0, .gap = 0});
+        QCOMPARE(rects[4].left(), 0.0);
+        QCOMPARE(rects[5].width(), 300.0);
+    }
+
+    void blocksShareUnitAndSitAtTheBottom()
+    {
+        const Block tall{{key(), key()}, {key(), key()}};
+        const Block shortBlock{{key()}};
+        const auto rects = computeKeyRects({tall, shortBlock}, {400, 100}, {.padding = 0, .gap = 0, .blockGap = 100});
+        // 3 units in 300 px: 100 each.
+        QCOMPARE(rects[1].right(), 200.0);
+        QCOMPARE(rects[4], QRectF(300, 50, 100, 50));
+    }
 };
 
 QTEST_GUILESS_MAIN(TestKeyGeometry)