[poincare] HorizontalLayout is not collapsable.

This fixes a bug when pressing (^2) on an empty function definition in
Graph.

Change-Id: If9c07218d3aa7fe8a67b32c6f20d2e52800877b9

poincare/src/layout/horizontal_layout.h

@@ -39,6 +39,7 @@ public:
   /* Other */
   bool isHorizontal() const override { return true; }
   bool isEmpty() const override;
+  bool isCollapsable(int * numberOfOpenParenthesis, bool goingLeft) const override { return false; }
 
 protected:
   void render(KDContext * ctx, KDPoint p, KDColor expressionColor, KDColor backgroundColor) override;